U.S. patent application number 11/526692 was filed with the patent office on 2007-01-25 for optical receiving station, optical communication system, and dispersion controlling method.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Yuichi Akiyama, George Ishikaw, Hiroki Ooi.
Application Number | 20070019967 11/526692 |
Document ID | / |
Family ID | 18663138 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070019967 |
Kind Code |
A1 |
Akiyama; Yuichi ; et
al. |
January 25, 2007 |
Optical receiving station, optical communication system, and
dispersion controlling method
Abstract
The invention relates to an optical receiving station, an
optical communication system, and a dispersion controlling method
for precisely controlling chromatic dispersion in an optical
transmission line or chromatic dispersion in an optical
transmission line that varies with time. An optical receiving
station is provided with a dispersion compensating section for
receiving, via an optical transmission line, an optical signal
modulated according to an optical duo-binary modulation method and
for changing a dispersion value to be used for compensating for
chromatic dispersion in an optical transmission line, an intensity
detecting section for detecting the intensity of a specific
frequency component of the optical signal output from the
dispersion compensating section, and a controlling section for
adjusting the dispersion value of the dispersion compensating
section so that the output of the intensity detecting section has a
predetermined extreme value.
Inventors: |
Akiyama; Yuichi; (Kawasaki,
JP) ; Ooi; Hiroki; (Kawasaki, JP) ; Ishikaw;
George; (Kawasaki, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
18663138 |
Appl. No.: |
11/526692 |
Filed: |
September 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11300316 |
Dec 15, 2005 |
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11526692 |
Sep 26, 2006 |
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09790695 |
Feb 23, 2001 |
7006770 |
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11300316 |
Dec 15, 2005 |
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Current U.S.
Class: |
398/159 |
Current CPC
Class: |
H04B 10/25133 20130101;
H04B 10/66 20130101; H04B 10/25137 20130101; H04B 2210/252
20130101 |
Class at
Publication: |
398/159 |
International
Class: |
H04B 10/00 20060101
H04B010/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2000 |
JP |
2000-158708 |
Claims
1. A dispersion controlling method controlling chromatic dispersion
of an optical duo-binary signal transmitted to an optical
transmission line, comprising: detecting intensity of a specific
frequency component of said optical duo-binary signal; and
adjusting a total dispersion amount of said optical transmission
line so that the intensity detected has a predetermined extreme
value.
2. A dispersion controlling apparatus controlling chromatic
dispersion of an optical duo-binary signal transmitted to an
optical transmission line, comprising: means for detecting
intensity of a specific frequency component of said optical
duo-binary signal; and means for adjusting a total dispersion
amount of said optical transmission line so that the intensity
detected has a predetermined extreme value.
Description
[0001] This application is a divisional application of Ser. No.
11/300,316, filed Dec. 15, 2005, currently pending, which is a
divisional of Ser. No. 09/790,695, filed Feb. 23, 2001, which is
now U.S. Pat. No. 7,006,770, issued Feb. 28, 2006.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical receiving
station and an optical communication system for compensating for
chromatic dispersion in an optical transmission line in an optical
transmission system where an optical duo-binary modulation method
is used. The invention also relates to a dispersion compensating
method for adjusting the total dispersion amount of an optical
transmission line.
[0004] At present, optical communication apparatuses capable of
transmitting a large amount of optical signals over an ultra-long
distance are required for construction of future multimedia
networks. Optical transmission systems of 10 Gb/s have been put in
practical use in current trunk line optical communication, to
satisfy the above requirement. Further, time division multiplexing
optical transmission systems of 40 Gb/s have been studied and
developed.
[0005] 2. Description of the Related Art
[0006] The maximum transmission distance of an optical fiber
without relaying is limited by attenuation and chromatic dispersion
of an optical signal. To increase the transmission distance, it is
necessary to compensate for the attenuation and chromatic
dispersion. The attenuation is compensated by a
rare-earth-element-doped fiber amplifier or the like. On the other
hand, the chromatic dispersion is compensated by inserting, in an
optical transmission line, a dispersion compensator having a fixed
dispersion characteristic of an opposite sign to the sign of the
value of dispersion received by an optical signal traveling through
an optical fiber.
[0007] Incidentally, being in inverse proportion to the square of
the bit rate of an optical signal, the dispersion compensation
tolerance at 40 Gb/s is as small as 1/16 of that at 10 Gb/s. This
makes it necessary to precisely adjust the dispersion value of a
dispersion compensator.
[0008] The dispersion compensation tolerance signifies the width of
an allowable dispersion compensation value range for satisfaction
of a certain transmission condition when compensating for chromatic
dispersion by a dispersion compensator. For example, it is an
allowable dispersion compensation value range for suppressing the
power penalty (i.e., deterioration in the receiver sensitivity of
an optical signal due to transmission) to 1 dB or less.
[0009] The chromatic dispersion of an optical transmission line
varies with time due to a temperature variation, for example. The
variation amount of dispersion of an optical transmission line is
given by (temperature dependence of the zero dispersion wavelength
of the optical transmission line).times.(temperature
variation).times.(dispersion slope).times.(transmission distance).
For example, where the optical transmission line is a
dispersion-shifted optical fiber, the temperature variation is
-40.degree. C. to +60.degree. C., and the transmission distance is
600 km, 0.03 nm/.degree. C..times.100.degree. C..times.0.08
ps/nm.sup.2/km.times.600 km=144 ps/nm.
[0010] This value is not negligible even if an optical duo-binary
modulation method with a wide dispersion compensation tolerance is
employed to modulate an optical signal.
[0011] A simulation shows that with a possible transmission
condition of the eye pattern penalty 1 dB or less, the dispersion
compensation tolerance of an optical duo-binary modulation method
of 40 Gb/s is 400 ps/nm, which is 22 km (400 ps/nm+18.6 ps/nm/km)
in terms of the length of an existing single-mode optical
fiber.
SUMMARY OF INVENTION
[0012] An object of the present invention is therefore to provide
an optical receiving station, an optical communication system, and
a dispersion controlling method for precisely controlling chromatic
dispersion in an optical transmission line for transmitting an
optical signal in an optical transmission system where an optical
duo-binary modulation method is used.
[0013] Another object of the invention is to provide an optical
receiving station, an optical communication system, and a
dispersion controlling method for precisely controlling chromatic
dispersion in an optical transmission line that varies with time in
an optical transmission system where an optical duo-binary
modulation method is employed.
[0014] An optical duo-binary modulation method and modulator are
disclosed in Japanese Unexamined Patent Application Publication No.
Hei08-139681 and Japanese National Patent Publication No.
Hei09-501296, for example.
[0015] The above objects are attained by the following
sections.
[0016] According to a first section of the invention, an optical
receiving station comprises a dispersion compensating section for
receiving, via an optical transmission line, an optical signal
modulated according to an optical duo-binary modulation method, and
for changing a dispersion value to be used for compensating for
chromatic dispersion in an optical transmission line; an intensity
detecting section for detecting intensity of a specific frequency
component of the optical signal output from the dispersion
compensating section; and a controlling section for adjusting the
dispersion value of the dispersion compensating section so that an
output of the intensity detecting section has a predetermined
extreme value.
[0017] According to a second section of the invention, an optical
receiving station comprises a filter for receiving, via an optical
transmission line, an optical signal modulated according to an
optical duo-binary modulation method, and for changing a passing
wavelength range to be used for compensating for chromatic
dispersion in an optical transmission line; an intensity detecting
section for detecting intensity of a specific frequency component
of the optical signal output from the filter; and a wavelength
controlling section for adjusting a wavelength of the optical
signal so that an output of the intensity detecting section has a
predetermined extreme value, and for adjusting the passing
wavelength range of the filter to pass the adjusted wavelength.
[0018] According to a third section of the invention, in the
optical receiving station according to the first or second section,
a dispersion compensator for compensating for the chromatic
dispersion in the optical transmission line with a fixed dispersion
value is further provided and the optical signal is input to the
intensity detecting section via the dispersion compensator.
[0019] According to a fourth section of the invention, an optical
communication system comprises an optical sending station for
generating an optical signal according to an optical duo-binary
modulation method; an optical transmission line for transmitting
the generated optical signal; and the optical receiving station
according to one of the first to third sections for receiving the
transmitted optical signal.
[0020] According to a fifth section of the invention, a dispersion
controlling method for controlling chromatic dispersion in an
optical transmission line for transmitting an optical signal
modulated according to an optical duo-binary modulation method
comprises the steps of detecting intensity of a specific frequency
component of the optical signal; and adjusting a total dispersion
amount of the optical transmission line so that the detected
intensity has a predetermined extreme value.
[0021] For instance, the receiver sensitivity of an optical signal
can be evaluated according to the eye aperture of an eye pattern. A
certain relationship holds between the eye aperture characteristic
with respect to a variance of the total chromatic dispersion of an
optical transmission line and the intensity characteristic of a
specific frequency component with respect to a variance of the
chromatic dispersion.
[0022] Therefore, in the above optical receiving station, the
optical communication system, and the dispersion compensating
method, it is possible to precisely control the total chromatic
dispersion since the total dispersion amount of an optical
transmission line is controlled by a variable dispersion
compensating section in accordance with the intensity of a
detectable specific frequency component without directly detecting
the total chromatic dispersion of the optical transmission line.
The precise control of the total chromatic dispersion enables
optimization of the receiver sensitivity and long distance
transmission. Further, the above optical the receiving station, the
optical communication system, and the dispersion compensating
method can cope with variations with time because the dispersion
value of the dispersion compensating section is variable.
[0023] The invention makes it possible to precisely compensate an
optical signal modulated according to an optical duo-binary
modulation method for chromatic dispersion in an optical
transmission line. The invention also makes it possible to
precisely compensate for chromatic dispersion in an optical
transmission line that varies with time. Therefore, the
transmission distance of can be increased in an optical
communication system according to the invention. Further, the
invention enables effective use of existing optical communication
networks with 1.3-.mu.m-band, single-mode optical fibers.
[0024] Other preferred sections for attaining the objects will be
described in the following embodiments of the invention and the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The nature, principle, and utility of the invention will
become more apparent from the following detailed description when
read in conjunction with the accompanying drawings in which like
parts are designated by identical reference numbers, in which:
[0026] FIGS. 1A and 1B show the configurations of optical
communication systems according to a first embodiment of the
present invention;
[0027] FIG. 2 shows the configuration of an optical sending station
of the optical communication system according to the first
embodiment;
[0028] FIG. 3 shows the configuration of an optical receiving
station of the optical communication system according to the first
embodiment;
[0029] FIG. 4 shows one exemplary configuration of a variable
dispersion compensator;
[0030] FIGS. 5A and 5B are graphs showing voltage patterns to be
applied to respective segments of the variable dispersion
compensator and a dispersion characteristic for each voltage
pattern, respectively;
[0031] FIG. 6 is a graph showing a dispersion compensation method
for a case where no nonlinear optical effect occurs in light
transmission;
[0032] FIG. 7 is a graph showing a dispersion compensation method
for a case where a nonlinear optical effect occurs in light
transmission;
[0033] FIGS. 8A-8C are graphs showing an intensity vs. total
chromatic dispersion characteristics and an eye aperture vs. total
chromatic dispersion characteristic in a linear range;
[0034] FIGS. 9A-9C are graphs showing an intensity vs. total
chromatic dispersion characteristics and an eye aperture vs. total
chromatic dispersion characteristic in a nonlinear range;
[0035] FIG. 10 shows the configuration of a modified optical
receiving station that is used in the optical communication system
according to the first embodiment;
[0036] FIGS. 11A and 11B show the configurations of optical
communication systems according to a second embodiment of the
invention;
[0037] FIG. 12 shows the configuration of an optical sending
station of the optical communication system according to the second
embodiment;
[0038] FIG. 13 shows the configuration of an optical receiving
station of the optical communication system according to the second
embodiment; and
[0039] FIGS. 14A and 14B each show an intensity vs. wavelength
characteristic and an eye aperture vs. wavelength characteristic;
and
[0040] FIG. 15 shows the configuration of a modified optical
receiving station that is used in the optical communication system
according to the second embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] Embodiments of the present invention will be hereinafter
described with reference to the accompanying drawings. In the
drawings, the same components are given the same reference symbols
and descriptions therefor may be omitted.
First Embodiment
(Configuration)
[0042] A first embodiment will be described with reference to FIGS.
1A and 1B to FIG. 10. The first embodiment is directed to an
optical sending station, an optical communication system, and a
dispersion controlling method according to the invention.
[0043] As shown in FIG. 1A, an optical communication system
according to the first embodiment is composed of an optical sending
station 11, an optical transmission line 12, and an optical
receiving station 14.
[0044] An optical signal generated by the optical sending station
11 through modulation according to an optical duo-binary modulation
method is sent to the optical transmission line 12, subjected to
attenuation and chromatic dispersion in the optical transmission
line 12, and then subjected to reception processing in the optical
receiving station 14.
[0045] Where the transmission distance between the optical sending
station 11 and the optical receiving station 14 is long, a
necessary number of repeater stations 13 are provided in the
optical transmission line 12 as shown in FIG. 1B. Having an optical
amplifier etc., each repeater station 13 amplifies and relays an
optical duo-binary signal. Examples of the optical amplifier are a
semiconductor optical amplifier and a rare-earth-element-doped
fiber amplifier.
[0046] Next, the configuration of the optical sending station 11
used in this optical communication system will be described with
reference to FIG. 2.
[0047] The optical sending station 11 is composed of an NRZ
generator 101, a precoder 102, a D flip-flop (hereinafter
abbreviated as "D-FF") 103, amplifiers 104-a and 104-b, low-pass
filters (hereinafter abbreviated as "LPF") 105-a and 105-b, a
semiconductor laser (hereinafter abbreviated as "LD") 106, and a
Mach-Zehnder interferometer type optical modulator (hereinafter
abbreviated as "MZ modulator") 107.
[0048] The NRZ generator 101 generates a non-return-to-zero
(hereinafter abbreviated as "NRZ") binary electrical signal that is
in accordance with information to be transmitted from the optical
sending station 11 to the optical receiving station 14.
[0049] The generated NRZ signal is input to an inverter 111 of the
precoder 102. The precoder 102 is composed of the inverter 111, an
exclusive-OR circuit (hereinafter abbreviated as "EXOR") 112, and a
delay circuit 113.
[0050] The inverter 111 inverts an NRZ signal, that is, changes the
truth value "0" to "1" and "1" to "0," and outputs the inverted NRZ
signal to one port of the EXOR 112. For example, in the case of
positive logic, the inverter 111 inverts a high voltage level to a
low voltage level and a low voltage level to a high voltage level,
and outputs a resulting signal to the one port of the EXOR 112.
[0051] An output of the EXOR 112 is input to the delay circuit 113
and the D-FF 103. The delay circuit 113 delays the input signal by
1 bit and outputs the delayed signal to the other port of the EXOR
112.
[0052] Therefore, EXOR 112 exclusive-ORs the outputs of the
inverter 111 and the delay circuit 113 and outputs a resulting
signal.
[0053] The D-FF 103 delays a received signal by a one-clock period
and outputs a resulting signal. An output Q is amplified by the
amplifier 104-a and then applied to one electrode 117-a of the MZ
modulator 107 via the LPF 105-a. An inverted output Q is amplified
by the amplifier 104-b and then applied to the other electrode
117-b of the MZ modulator 107 via the LPF 105-b.
[0054] The precoder 102, the D-FF 103, the amplifiers 104, and the
LPFs 105 convert a binary NRZ signal into a ternary, duo-binary
signal.
[0055] The MZ modulator 107 has electrodes 117 and an optical
waveguide 116 that is formed by thermally diffusing titanium (Ti)
in a lithium niobate (LiNbO.sub.3) substrate. The optical waveguide
116 divides halfway into two branches, the electrodes 117 are
placed on the respective branches of the optical waveguide 116, and
the branches merge with each other on the output side.
[0056] The LD 106 emits laser light, or an optical carrier wave.
The laser light is input to the MZ modulator 107, where it is
modulated in light intensity according to a duo-binary signal
applied to the electrodes 117 and thereby becomes an optical
duo-binary signal, which is output to the optical transmission line
12.
[0057] Next, the configuration of the optical receiving station 14
that is used in this optical communication system will be described
with reference to FIG. 3.
[0058] The optical receiving station 14 is composed of a dispersion
compensating part 21, a coupler 122 for branching incident light
into two parts, an optical receiving part 123, an intensity
detecting part 22, and a controlling part 23. The dispersion
compensating part 21 can receive an optical duo-binary signal and
change a dispersion value to be used for compensating for chromatic
dispersion. The intensity detecting part 22 detects the intensity
of a specific frequency component of an optical duo-binary signal.
The controlling part 23 adjusts the dispersion value of the
dispersion compensating part 21 so that the output of the intensity
detecting part 22 has a predetermined extreme value.
[0059] A more detailed description will be made below.
[0060] An optical duo-binary signal transmitted by the optical
transmission line 12 is input to a variable dispersion compensator
(hereinafter abbreviated as "VDC") 121 of the dispersion
compensating part 21.
[0061] The VDC 121 compensates for chromatic dispersion of an
optical duo-binary signal by using a dispersion value according to
a control described below, and outputs a resulting signal to the
optical receiving part 123 and the intensity detecting part 22 via
the coupler 122.
[0062] The optical receiving part 123 receives and processes an
optical duo-binary signal and extracts information that was sent
from the optical sending station 11. For example, the optical
receiving part 123 can demodulate an optical duo-binary signal into
a binary electrical signal by detecting and photoelectrically
converting the optical duo-binary signal, supplying a resulting
ternary electrical signal to two discriminators for discriminating
between 1s and 0s, and exclusive-ORing outputs of the respective
discriminators.
[0063] On the other hand, the intensity detecting part 22 is
composed of a photodiode (hereinafter abbreviated as "PD") 124, a
band-pass filter (hereinafter abbreviated as "BPF") 125, an
amplifier 126, and a power meter 127.
[0064] An optical duo-binary signal that is input to the intensity
detecting part 22 is detected and photoelectrically converted by
the PD 124. The BPF 125 extracts only a 40 GHz frequency component
from a resulting electrical signal. An output of the BPF 125 is
amplified to a predetermined level by the amplifier 126 and the
power (intensity) of the amplified signal is detected by the power
meter 127.
[0065] An output of the power meter 127 is input to a central
processing unit (hereinafter abbreviated as "CPU") 128 of the
controlling part 23. The controlling part 23 has the CPU 128 and a
memory 129.
[0066] A table showing a relationship between voltage patterns and
dispersion values of the VDC 121, programs for operation of the CPU
128, etc. are stored in advance in the memory 129. Various values
etc. that occur during execution of a program are stored in the
memory 129 on each occasion. The memory 129 refers to the
relationship table etc. in response to a request from the CPU 128
and outputs a result to the CPU 128.
[0067] Being a microprocessor or the like, the CPU 128 outputs a
signal to be used for controlling the dispersion value of the VDC
121 to a driving circuit 130 of the dispersion compensating part
21.
[0068] The dispersion compensating part 21 is composed of the VDC
121 and the driving circuit 130 for driving the VDC 121. The
driving circuit 130 applies, to the VDC 121, voltages having a
voltage pattern that is in accordance with a signal that is output
from the CPU 128 and thereby changes the dispersion value of the
VDC 121.
[0069] Next, one exemplary configuration of the dispersion
compensating part 21 will be described.
[0070] The dispersion compensating part 21 is composed of the VDC
121 and the driving circuit 130.
[0071] As shown in FIG. 4, the VDC 121 is configured in such a
manner that piezoelectric elements 142 are attached to 21
respective segments of a chirped fiber Bragg grating 141. When
voltages that are graded as shown in FIG. 5A are applied, as
application voltages V1-V21 to be supplied to the respective
piezoelectric elements 142, to the VDC 121, the pressure acting on
the chirped fiber Bragg grating 141 in the longitudinal direction
varies. The dispersion value (slope of a line) is varied as shown
in FIG. 5B for voltage patterns A-D shown in FIG. 5A.
[0072] FIG. 4, FIGS. 5A and 5B, and the above related descriptions
are excerpts from M. M. Ohm et al., "Tunable Fiber Grating
Dispersion Using a Piezoelectric Stack," OFC '97 Technical Digest,
WJ3, pp. 155-156.
[0073] Another, simplified version of the dispersion compensating
part 21 is known that is composed of a plurality of dispersion
compensation fibers having different dispersion compensation
amounts, an optical switch, and a controlling CPU for controlling
the optical switch. The dispersion compensation amount is varied
discontinuously by selecting a dispersion compensation fiber with
the optical switch.
[0074] Among other methods for implementing a variable dispersion
compensator are a method in which the dispersion value is varied by
giving a temperature gradient to a fiber grating (Sergio Brarcelos
et al., "Characteristics of Chirped Fiber Gratings for Dispersion
Compensation," OFC '96 Technical Digest, WK12, pp. 161-162) and a
method in which the dispersion value is varied by a
temperature-variation-induced phase variation to a PLC (planar
lightwave circuit) (K. Takiguchi et al., "Variable Group-delay
Dispersion Equalizer Using Lattice-form Programmable Optical Filter
on Planar Ughtwave Circuit," IEEE J. Selected Topics in Quantum
Electronics, 2, 1996, pp. 270-276).
Functions and Advantages of the First Embodiment
[0075] First, the basic concept will be described.
[0076] FIG. 6 illustrates a dispersion compensation method for a
case where no nonlinear optical effect occurs in light
transmission. In FIG. 6, the horizontal axis represents the total
chromatic dispersion in ps/nm, the left vertical axis represents
the intensity, and the right vertical axis represents the eye
aperture. The intensity is absolute intensity and the eye aperture
is a value that is normalized by the maximum vertical aperture of
the eye of an eye pattern.
[0077] FIG. 6 includes two characteristic curves with respect to
the total chromatic dispersion. The lower solid-line curve
represents an intensity characteristic of a 40 GHz frequency
component of an optical duo-binary signal. The upper solid-line
curve represents an eye aperture characteristic. FIGS. 7-9 are
drawn in the same manner as FIG. 6.
[0078] The curves in FIG. 6 show results of a simulation in which
input light having an average optical power of 0 dBm is input to a
single-mode optical fiber (SMF) and transmitted through it over 50
km.
[0079] As shown in FIG. 6, the eye aperture characteristic has a
minimal value at a total chromatic dispersion value of 0 ps/nm and
maximal values (also greatest values in the case of FIG. 6) on the
left and right thereof. On the other hand, the intensity
characteristic has a maximal value (also a greatest value in the
case of FIG. 6) at a total chromatic dispersion value of 0 ps/nm.
This results from the facts that the optical signal is of a
duo-binary modulation type and the input optical power of the
optical signal is not large enough to cause a nonlinear optical
effect, and other factors.
[0080] Therefore, the relationship as shown in FIG. 6 holds between
the eye aperture characteristic that relates to deterioration in
receiver sensitivity and the intensity characteristic of the
specific frequency component of an optical signal that are coupled
with each other via the total chromatic dispersion.
[0081] If it is assumed that the reception condition is that the
eye aperture should be higher than or equal to the level of point a
that is lower than or equal to the minimal value, it is proper that
the total chromatic dispersion N be in a range of
n1.ltoreq.N.ltoreq.n9. Therefore, if the chromatic dispersion is
compensated for by a variable dispersion compensator so that the
intensity of the specific frequency component is always equal to
the greatest value, the total chromatic dispersion becomes 0 ps/nm.
At this time, the eye aperture is higher than or equal to the level
of point a and hence the reception condition is satisfied.
[0082] If it is assumed that the reception condition is that the
eye aperture should be higher than or equal to the level of point b
that is higher than the minimal value, it is proper that the total
chromatic dispersion N be in ranges of n2.ltoreq.N.ltoreq.n4 and
n6.ltoreq.N.ltoreq.n8. In this case, if the chromatic dispersion of
a variable dispersion compensator is so adjusted that the intensity
of the specific frequency component is kept equal to the greatest
value, the eye aperture becomes lower than or equal to the level of
point b. Therefore, to satisfy, as in the above case, the reception
condition by adjusting the chromatic dispersion of a variable
dispersion compensator so that the intensity of the specific
frequency component is always equal to the greatest value, a total
chromatic dispersion value where the intensity of the specific
frequency component has a greatest value may be shifted by using a
dispersion compensator having a fixed dispersion compensation
value.
[0083] That is, the intensity of the specific frequency component
is detected via a fixed dispersion compensator and the chromatic
dispersion is compensated for by a variable dispersion compensator
so that the detected intensity is always equal to the greatest
value. With this measure, the eye aperture becomes higher than or
equal to the level of point b and hence the reception condition is
satisfied. The fixed chromatic dispersion value may be set in a
range of n6-n8 (or n2-n4).
[0084] In particular, if a fixed dispersion compensator having a
chromatic dispersion value where the eye aperture has the greatest
value, that is, a chromatic dispersion value of point n7 (or n3),
the eye aperture is made equal to the greatest value and hence the
reception condition is satisfied in optimum form.
[0085] Next, a description will be made of a case where the optical
power of input light is so large that an optical signal experiences
a nonlinear optical effect in an optical transmission line.
[0086] FIG. 7 illustrates a dispersion compensation method for a
case where a nonlinear optical effect occurs in light
transmission.
[0087] Two curves in FIG. 7 are results of a simulation in which
input light having an average optical power of +15 dBm is input to
a single-mode optical fiber and transmitted through it over 50
km.
[0088] As shown in FIG. 7, the eye aperture characteristic has a
greatest value when the total chromatic dispersion is about 30
ps/nm. As the total chromatic dispersion increases in a range of
about 30-110 ps/nm, the eye aperture varies gently and is
approximately kept at the greatest level. On the other hand, the
intensity characteristic has a plurality of extreme values. This
results from the facts that the optical signal is of a duo-binary
modulation type and the input optical power of the optical signal
is large enough to cause a nonlinear optical effect, and other
factors.
[0089] Therefore, the relationship as shown in FIG. 7 holds between
the eye aperture characteristic that relates to deterioration in
receiver sensitivity and the intensity characteristic of the
specific frequency component of an optical signal that are coupled
with each other via the total chromatic dispersion.
[0090] If it is assumed that the reception condition is that the
eye aperture should be higher than or equal to the level of point
c, it is proper that the total chromatic dispersion N be in a range
of m1.ltoreq.N.ltoreq.m4. On the other hand, in this range, the
intensity of the specific frequency component has a minimal value
at m2 and a maximal value (also a greatest value in the case of
FIG. 7) at m3. Therefore, if the chromatic dispersion is
compensated for by a variable dispersion compensator so that the
intensity of the specific frequency component is always equal to
the minimal value, the eye aperture is higher than or equal to the
level of point c and hence the reception condition is satisfied.
Alternatively, if the chromatic dispersion is compensated for by a
variable dispersion compensator so that the intensity of the
specific frequency component is always equal to the greatest value,
the eye aperture is higher than or equal to the level of point c
and hence the reception condition is satisfied.
[0091] As described above, for each of various optical power values
of input light, an intensity characteristic of a specific frequency
component and an eye aperture characteristic with respect to the
total chromatic dispersion are determined in advance for a
predetermined optical transmission line by measurements or
simulations. Then, the chromatic dispersion is compensated for by a
dispersion compensation method that is selected from the following
methods (1)-(3) in accordance with the input optical power of an
optical signal and a reception condition (eye aperture condition)
in a target optical communication system:
[0092] (1) The intensity of a specific frequency component of an
optical signal is detected and the total chromatic dispersion
amount of the optical transmission line is adjusted by a variable
dispersion compensator so that the detected intensity is always
equal to a maximal value.
[0093] In specific, at input optical power that causes a nonlinear
optical effect, a plurality of maximal values exist in the
intensity characteristic. Therefore, a maximal value is selected in
a total chromatic dispersion range in which the eye aperture
satisfies the reception condition.
[0094] (2) The intensity of a specific frequency component of an
optical signal is detected via a dispersion compensator having a
fixed dispersion value, and the total chromatic dispersion amount
of the optical transmission line is adjusted by a variable
dispersion compensator so that the detected intensity is always
equal to a maximal value.
[0095] In particular, from the viewpoint of obtaining best receiver
sensitivity, it is preferable that the fixed dispersion value is
set equal to the difference between a total chromatic dispersion
amount where the intensity of the specific frequency component has
a greatest value and a total chromatic dispersion amount where the
eye aperture has a greatest value.
[0096] (3) The intensity of a specific frequency component of an
optical signal is detected and the total chromatic dispersion
amount of the optical transmission line is adjusted by a variable
dispersion compensator so that the detected intensity is always
equal to a minimal value.
[0097] In particular, at input optical power that causes a
nonlinear optical effect, a plurality of minimal values exist in
the intensity characteristic. Therefore, a minimal value is
selected in a total chromatic dispersion range in which the eye
aperture satisfies the reception condition.
[0098] The frequency of a specific frequency component is set equal
to the bit rate of an optical signal. For example, where the bit
rate of an optical duo-binary modulation signal is 40 Gb/s as in
this embodiment, a 40 GHz frequency component of this optical
duo-binary modulation signal is employed as a specific frequency
component.
[0099] FIGS. 8A-8C and 9A-9C show results of simulations in which
input light beams having average optical power values Pin of 0 dBm,
+3 dBm, +6 dBm, +9 dBm, +12 dBm, and +15 dBm are input to a
single-mode optical fiber and transmitted through it over 50 km. In
this case, 0 dBm, +3 dBm, and +6 dBm are input optical power values
that are not sufficient to cause a nonlinear optical effect and +9
dBm, +12 dBm, and +15 dBm are input optical power values large
enough to cause a nonlinear optical effect.
[0100] In FIGS. 8A-8C and 9A-9C, marks ".tangle-solidup.",
".box-solid.", and ".DELTA.", which exist in ranges where the eye
aperture is sufficiently large, indicate a maximal value of the
intensity of a 40 GHz frequency component, a minimal value of the
intensity of a 40 GHz frequency component, and a greatest value of
the eye aperture, respectively.
[0101] Next, the functions and the advantages of this embodiment
will be described in more detail.
[0102] First, a description will be made of a case where the
average input optical power of an optical signal is +3 dBm and the
eye aperture is 0.7 and the dispersion compensation method (1) is
employed.
[0103] First, in installing an optical communication system that is
composed of the optical sending station 11, the optical
transmission line 12, the optical receiving station 14, and, if
necessary, the repeater stations 13, an installation party sets the
average input optical power of an optical signal that is sent from
the optical sending station 11 at +3 dBm.
[0104] The optical sending station 11 sends out an optical
duo-binary signal. A test optical duo-binary signal may be sent
out.
[0105] The optical signal thus sent is transmitted by the optical
transmission line 12 and received by the optical receiving station
14. The intensity detecting part 22 of the optical receiving
station 14 detects the intensity of a 40 GHz frequency
component.
[0106] The CPU 128 varies the dispersion value of the VDC 121 from
the smallest value to the greatest value at first constant
intervals. For each constant interval, an output of the intensity
detecting part 22 is stored in the memory 129 together with a
dispersion value (voltage pattern) at that time. Outputs of the
intensity detecting part 22 are represented by D0, D1, D2, D3, . .
. , Dj.
[0107] The CPU 128 determines a greatest value from D0, D1, D2, D3,
. . . , Dj that are stored in the memory 129. The greatest value is
represented by Dmax0.
[0108] The CPU 128 searches the memory 129 for a dispersion value
(voltage pattern) corresponding to the greatest value, and adjusts
the DVC 121 so that it provides the dispersion value thus found.
The CPU 128 stores the greatest value Dmax0 in the memory 129.
[0109] The CPU 128 again varies the dispersion value of the VDC 121
in the increasing direction at second constant intervals that are
smaller than the first constant intervals and captures outputs of
the intensity detecting part 22. The greatest value of those
outputs is represented by Dmax1.
[0110] The CPU 128 compares Dmax0 and Dmax1.
[0111] If Dmax1 is greater than Dmax0, the CPU 128 makes Dmax1 a
new Dmax0 and again varies the dispersion value of the VDC 121 in
the increasing direction at the second constant intervals. The
above operation is repeated until Dmax1 becomes smaller than Dmax0,
whereby a greatest value can be detected precisely in a case where
Dmax0 is located on an intensity characteristic curve having a
positive slope. Even when the optical communication system is in
service, the above operations may be performed to cope with
variations with time.
[0112] On the other hand, if Dmax1 is smaller than Dmax0, the CPU
128 does not make Dmax1 a new Dmax0 and varies the dispersion value
of the VDC 121 in the decreasing direction at the second constant
intervals from the dispersion value (voltage pattern) corresponding
to Dmax0. The CPU 128 captures outputs of the intensity detecting
part 22. The greatest value of those outputs is represented by
Dmax2.
[0113] The CPU 128 compares Dmax0 and Dmax2.
[0114] If Dmax2 is greater than Dmax0, the CPU 128 makes Dmax2 a
new Dmax0 and again varies the dispersion value of the VDC 121 in
the decreasing direction at the second constant intervals. By
repeating the above operation until Dmax2 becomes smaller than
Dmax0, a greatest value can be detected precisely in a case where
Dmax0 is located at an intensity characteristic curve having a
negative slope. Even when the optical communication system is in
service, the above operations may be performed to cope with
variations with time.
[0115] Using two kinds of intervals, that is, the first and second
intervals, the CPU 128 can detect a greatest value quickly and
precisely.
[0116] Although the above description is directed to the case where
the input optical power of an optical signal is +3 dBm, chromatic
dispersion can be compensated for in a similar manner for any input
optical power such as 0 dBm, +6 dBm, +9 dBm, +12 dBm, or +15 dBm.
Where there exist a plurality of maximal values as in the cases of
+9 dBm, +12 dBm, or +15 dBm, data indicating where the target
maximal value stands in the succession of maximal values in the
variation range of total chromatic dispersion is stored in the
memory 129 to allow the CPU 128 to extract the target maximal value
by referring to the data. Alternatively, the dispersion variation
range of the VDC 121 is limited so that the variation range of
total chromatic dispersion is restricted so as to include only the
target maximal value to allow the CPU 128 to extract the target
maximal value. In this manner, the CPU 128 is enabled to extract
only the target maximal value among a plurality of maximal
values.
[0117] Next, a description will be made of a case where the average
input optical power of an optical signal is +3 dBm and the eye
aperture is 0.7 and the dispersion compensation method (2) is
employed. Where the eye aperture is 0.85, for example, chromatic
dispersion cannot be compensated for by the dispersion compensation
method (1) and hence it is necessary to employ the dispersion
compensation method (2).
[0118] First, a configuration of the optical receiving station 24
that is employed in this case will be described.
[0119] FIG. 10 shows the configuration of a modified optical
receiving station that is used in the optical communication system
according to the first embodiment.
[0120] The optical receiving station 24 is the same as the optical
receiving station 14 shown in FIG. 3 except that a fixed dispersion
compensator (hereinafter abbreviated as "DC") 151 is provided
between the coupler 122 and the PD 124 as shown in FIG. 10, and
hence its configuration will not be described.
[0121] Further, the optical communication system having the optical
sending station 11, the optical transmission line 12, the optical
receiving station 24, and, if necessary, the repeater stations 13
have the same functions and advantages as that of method (1) and
hence its functions and advantages will not be described.
[0122] If the dispersion value of the DC 151 is set at about .+-.95
ps/nm in the case of FIG. 8A, about .+-.98 ps/nm in the case of
FIG. 8B, and about +95 ps/nm and -97 ps/nm in the case of FIG. 8C,
by always keeping the intensity at a greatest value the total
chromatic dispersion can be optimized so that the eye aperture is
equal to a greatest value.
[0123] Next, a description will be made of a case where the average
input optical power of an optical signal is +12 dBm and the eye
aperture is 0.8 and the dispersion compensation method (3) is
employed. In this case, dispersion compensation is also possible
with the dispersion compensation method (1).
[0124] First, in installing an optical communication system that is
composed of the optical sending station 11, the optical
transmission line 12, the optical receiving station 14, and, if
necessary, the repeater stations 13, an installation party sets the
average input optical power of an optical signal that is sent from
the optical sending station 11 at +12 dBm.
[0125] The optical sending station 11 sends out an optical
duo-binary signal.
[0126] The optical signal thus sent is transmitted by the optical
transmission line 12 and received by the optical receiving station
14. The intensity detecting part 22 of the optical receiving
station 14 detects the intensity of a 40 GHz frequency
component.
[0127] The CPU 128 varies the dispersion value of the VDC 121 from
the smallest value to the greatest value at first constant
intervals. For each constant interval, an output of the intensity
detecting part 22 is stored in the memory 129 together with a
dispersion value (voltage pattern) at that time. Outputs of the
intensity detecting part 22 are represented by D0, D1, D2, D3, . .
. , Dj.
[0128] The CPU 128 determines a smallest value from D0, D1, D2, D3,
. . . , Dj that are stored in the memory 129. The smallest value is
represented by Dmin0.
[0129] Where there exist a plurality of minimal values, data
indicating where the target minimal value stands in the succession
of minimal values in the variation range of total chromatic
dispersion is stored in the memory 129 to allow the CPU 128 to
extract the target maximal value by referring to the data.
Alternatively, the CPU 128 first detects a greatest value and then
extracts the target minimal value by using the greatest value as a
reference. For example, in the case of FIG. 9B, the target minimal
value is a minimal value that is found first when the total
chromatic dispersion is decreased from the total chromatic
dispersion value corresponding to the greatest value. As a further
alternative, the dispersion variation range of the VDC 121 is
limited so that the variation range of total chromatic dispersion
is restricted so as to include only the target minimal value to
allow the CPU 128 to extract the target maximal value. In this
manner, the CPU 128 is enabled to extract only the target minimal
value among a plurality of minimal values.
[0130] The CPU 128 searches the memory 129 for a dispersion value
(voltage pattern) corresponding to the minimal value, and adjusts
the DVC 121 so that it provides the dispersion value thus found.
The CPU 128 stores the minimal value Dmin0 in the memory 129.
[0131] The CPU 128 again varies the dispersion value of the VDC 121
in the increasing direction at second constant intervals that are
smaller than the first constant intervals and captures outputs of
the intensity detecting part 22. The smallest value of those
outputs is represented by Dmin1.
[0132] The CPU 128 compares Dmin0 and Dmin1.
[0133] If Dmin1 is smaller than Dmin0, the CPU 128 makes Dmin1 a
new Dmin0 and again varies the dispersion value of the VDC 121 in
the increasing direction at the second constant intervals. The
above operation is repeated until Dmin1 becomes greater than Dmin0,
whereby a minimal value can be detected precisely in a case where
Dmin0 is located on an intensity characteristic curve having a
negative slope. Even when the optical communication system is in
service, the above operations may be performed to cope with
variations with time.
[0134] On the other hand, if Dmin1 is greater than Dmin0, the CPU
128 does not make Dmin1 a new Dmin0 and varies the dispersion value
of the VDC 121 in the decreasing direction at the second constant
intervals from the dispersion value (voltage pattern) corresponding
to Dmin0. The CPU 128 captures outputs of the intensity detecting
part 22. The smallest value of those outputs is represented by
Dmin2.
[0135] The CPU 128 compares Dmin0 and Dmin2.
[0136] If Dmin2 is smaller than Dmin0, the CPU 128 makes Dmin2 a
new Dmin0 and again varies the dispersion value of the VDC 121 in
the decreasing direction at the second constant intervals. By
repeating the above operation until Dmin2 becomes greater than
Dmin0, a minimal value can be detected precisely in a case where
Dmin0 is located at an intensity characteristic curve having a
positive slope. Even when the optical communication system is in
service, the above operations may be performed to cope with
variations with time.
[0137] Although the above description is directed to the case where
the input optical power of an optical signal is +12 dBm, chromatic
dispersion can be compensated for in a similar manner for input
optical power that causes a nonlinear optical effect such as +9 dBm
or +15 dBm.
Second Embodiment
(Configuration)
[0138] A second embodiment will be described below with reference
to FIGS. 11A and 11B to FIG. 15. The second embodiment is directed
to an optical sending station, an optical communication system, and
a dispersion controlling method according to the invention. Whereas
in the first embodiment the total chromatic dispersion is optimized
by changing the chromatic dispersion value of the VDC 121, in the
second embodiment the total chromatic dispersion is optimized by
changing the wavelength of the optical carrier wave of an optical
duo-binary signal.
[0139] As shown in FIG. 11A, an optical communication system
according to the second embodiment is composed of an optical
sending station 41, an optical transmission line 12, and an optical
receiving station 44.
[0140] An optical duo-binary signal generated by the optical
sending station 41 is sent to the optical transmission line 12 and
then subjected to reception processing in the optical receiving
station 44. A line through which to transmit a control signal
(described later) from the optical receiving station 44 to the
optical sending station 41 is provided.
[0141] Where the transmission distance between the optical sending
station 41 and the optical receiving station 44 is long, a
necessary number of repeater stations 13 are provided in the
optical transmission line 12 as shown in FIG. 11B. Having an
optical amplifier etc., each repeater station 13 amplifies an
optical duo-binary signal.
[0142] The optical sending station 41 of the second embodiment is
the same as the optical sending station 11 of the first embodiment
shown in FIG. 2 except that as shown in FIG. 12 a
wavelength-tunable laser (hereinafter abbreviated as "t-LD") 161 is
used in place of the LD 106 and an LD controlling circuit 162 is
newly provided. Therefore, the configuration of the optical sending
station 41 will not be described except for the different
components.
[0143] The t-LD 161 is a semiconductor laser capable of changing
the oscillation wavelength such as a distributed Bragg reflector
(DBR) wavelength-tunable laser, a distributed feedback (DFB)
wavelength-tunable laser, a wavelength selection feedback
wavelength-tunable laser using an external diffraction grating, or
a composite resonator wavelength-tunable laser using an external
reflector.
[0144] The LD controlling circuit 162 receives a control signal to
be used for controlling the t-LD 161 so that it oscillates at a
predetermined wavelength that is transmitted from a CPU 168 of the
optical receiving station 44 via the above-mentioned line. The LD
controlling circuit 162 controls the t-LD 161 so that it oscillates
at the wavelength that is indicated by the received control signal.
For example, in the case of a DBR wavelength-tunable laser or a DFB
wavelength-tunable laser, the oscillation wavelength can be
controlled by changing the device temperature of the t-LD 161 with
a Peltier device or the like or by changing the injection current
of the t-LD 161 or by using both methods.
[0145] In the optical sending station 41, laser light having a
predetermined wavelength is emitted by the t-LD 161. As described
in the first embodiment, the laser light is input to the MZ
modulator 107, where it is modulated in light intensity according
to a duo-binary signal that is applied to the electrodes 117.
Modulated laser light is output to the optical transmission line 12
as an optical duo-binary signal.
[0146] On the other hand, the optical receiving station 44 of the
second embodiment is the same as the optical receiving station 14
of the first embodiment shown in FIG. 3 except that as shown in
FIG. 13 a filter part 51 and a controlling part 53 are provided in
place of the dispersion compensating part 21 and the controlling
part 23, respectively. Therefore, the configuration of the optical
receiving station 44 will not be described except for the different
components.
[0147] An optical duo-binary signal transmitted by the optical
transmission line 12 is input to a variable filter (hereinafter
abbreviated as "VFil") of the filter part 51.
[0148] The VFil 166 is a band-pass filter and can change its
passing wavelength range. The passing wavelength range is
controlled by the CPU 168 so as to pass the wavelength of the
optical duo-binary signal.
[0149] An optical signal that is output from the VFil 166 is
supplied to the optical receiving part 123 and the intensity
detecting part 22 via the coupler 122. An output of the intensity
detecting part 22 is input to the CPU 168 of the controlling
section 53. The controlling part 53 has the CPU 168 and a memory
169.
[0150] A table showing a relationship between passing wavelength
ranges of the VFil 166 and control signals, programs for operation
of the CPU 168, etc. are stored in advance in the memory 169.
Various values etc. that occur during execution of a program are
stored in the memory 169 on each occasion. The memory 169 returns a
result in response to a request from the CPU 168.
[0151] Being a microprocessor or the like, the CPU 168 outputs a
control signal to be used for controlling the passing wavelength
range of the VFil 166 to a VFil controlling circuit 167 of the
filter part 51 (details of control will be described below) and
also outputs a signal to be used for controlling the oscillation
wavelength of the t-LD 161 of the optical sending station 41 to the
LD controlling circuit 162.
[0152] The filter part 51 is composed of the VFil 166 and the VFil
controlling circuit 167 for driving the VFil 166. The VFil
controlling circuit 167 changes the passing wavelength range of the
VFil 166 according to a signal that is supplied from the CPU
168.
[0153] As described above, the optical receiving part 44 is
composed of the filter part 51, the coupler 122 for branching input
light into two parts, the optical receiving part 123, the intensity
detecting part 22, and the controlling part 53.
[0154] The filter part 51 can receive an optical duo-binary signal
and can change its own passing wavelength range. The intensity
detecting part 22 detects the intensity of a specific frequency
component of an optical duo-binary signal. The controlling part 53
adjusts the wavelength of an optical signal so that the output of
the intensity detecting part 22 has a predetermined extreme value,
and adjusts the passing wavelength range of the filter part 51 to
pass the thus-adjusted wavelength.
[0155] In the second embodiment, a control signal to be used for
adjusting the oscillation wavelength of the t-LD 161 is transmitted
from the optical receiving station 44 to the optical sending
station 41 via the dedicated physical line. However, the invention
is not limited to such a case. For example, in the case of an
optical wavelength division multiplexing signal, an optical signal
having one of its wavelengths may be used. Alternatively, undefined
bytes of a section overhead of SDH (synchronous digital hierarchy)
may be used. The section overhead is a portion for accommodating
information that is necessary for operation of a network, such as
maintenance information and a status monitor.
Functions and Advantages of the Second Embodiment
[0156] In the optical communication system according to the second
embodiment, the same effects as obtained in the first embodiment by
changing the dispersion value of the VDC 121 are obtained by
changing the wavelength of an optical signal. Therefore, the
functions and advantages of the second embodiment can be described
in the same manner as those of the first embodiment and hence will
not be described.
[0157] FIGS. 14A and 14B show exemplary relationships between the
intensity vs. wavelength characteristic and the eye aperture vs.
wavelength characteristic. FIG. 14A is obtained by converting the
total chromatic dispersion into the wavelength in FIG. 8A (input
optical power Pin=0 dBm). FIG. 14B is obtained by converting the
total chromatic dispersion into the wavelength in FIG. 9A (input
optical power Pin=+9 dBm). Figures corresponding to FIGS. 8B, 8C,
9B, and 9C can also be obtained by the same manner of
conversion.
[0158] As seen from FIGS. 14A and 14B, distribution compensation
methods similar to the above-described dispersion compensation
methods (1)-(3) can be used in the second embodiment. Therefore,
the total chromatic dispersion of the optical communication system
can also be optimized by the second embodiment.
[0159] A description will be made of the configuration of an
optical receiving station 64 in which the intensity of a specific
frequency component is detected via a fixed dispersion
compensator.
[0160] FIG. 15 shows the configuration of a modified optical
receiving station that is used in the optical communication system
according to the second embodiment.
[0161] The optical receiving station 64 is the same as the optical
receiving station 44 shown in FIG. 13 except that a DC 171 is
provided between the coupler 122 and the PD 124 as shown in FIG.
15, and hence its configuration will not be described. By using the
optical receiving station 64 in place of the optical receiving
station 44 shown in FIG. 11, a dispersion compensation method
similar to the above-described dispersion compensation method (2)
can be realized by changing the wavelength of an optical signal can
be realized.
[0162] Although the first and second embodiments are directed to
the optical communication systems that deal with a
single-wavelength optical duo-binary signal, the invention can also
be applied to a wavelength division multiplexing optical
communication system. That is, the invention may be practiced for
each wavelength component after a wavelength division multiplexing
optical signal is separated into different wavelength
components.
[0163] Although the first and second embodiments are directed to
the case where the specific frequency component has the frequency
of 40 GHz, the invention can also be applied to cases of other
frequency components because the specific relationship holds
between the intensity characteristic and the eye aperture
characteristic.
[0164] The invention is not limited to the above embodiments and
various modifications are possible without departing from the
spirit and scope of the invention. Any improvements may be made in
part or all of the components.
* * * * *