U.S. patent application number 11/992231 was filed with the patent office on 2009-05-28 for optical signal transmission control apparatus and optical signal transmission control method.
Invention is credited to Futoshi Izumi.
Application Number | 20090136239 11/992231 |
Document ID | / |
Family ID | 37942391 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090136239 |
Kind Code |
A1 |
Izumi; Futoshi |
May 28, 2009 |
Optical Signal Transmission Control Apparatus and Optical Signal
Transmission Control Method
Abstract
An optical signal transmission control apparatus that controls
transmission of optical signals transmitted via a plurality of
redundant routes. The optical signal transmission control apparatus
includes a delay difference adjusting unit that adjusts a
transmission delay difference between the optical signals of each
route by converting a wavelength of the optical signal and making
the optical signal with a converted wavelength pass through a
waveguide in which a transmission delay of the optical signal
changes continuously depending on the wavelength, and a waveform
degradation compensating unit that compensates degradation of a
waveform of the optical signal, while maintaining the transmission
delay difference adjusted by the delay difference adjusting
unit.
Inventors: |
Izumi; Futoshi; (Kawasaki,
JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700, 1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Family ID: |
37942391 |
Appl. No.: |
11/992231 |
Filed: |
September 30, 2005 |
PCT Filed: |
September 30, 2005 |
PCT NO: |
PCT/JP2005/018178 |
371 Date: |
March 19, 2008 |
Current U.S.
Class: |
398/147 |
Current CPC
Class: |
H04J 14/086
20130101 |
Class at
Publication: |
398/147 |
International
Class: |
H04B 10/12 20060101
H04B010/12 |
Claims
1. An optical signal transmission control apparatus that controls
transmission of optical signals transmitted via a plurality of
redundant routes, the optical signal transmission control apparatus
comprising: a delay difference adjusting unit that adjusts a
transmission delay difference between the optical signals of each
route by converting a wavelength of the optical signal and making
the optical signal with a converted wavelength pass through a
waveguide in which a transmission delay of the optical signal
changes continuously depending on the wavelength; and a waveform
degradation compensating unit that compensates degradation of a
waveform of the optical signal, while maintaining the transmission
delay difference adjusted by the delay difference adjusting
unit.
2. The optical signal transmission control apparatus according to
claim 1, further comprising a wavelength converting unit that
reconverts the wavelength of the optical signal to a predetermined
wavelength, after the wavelength of the optical signal is converted
by the delay difference adjusting unit.
3. The optical signal transmission control apparatus according to
claim 1, further comprising an optical buffer unit that adjusts the
transmission delay difference of the optical signals, by switching
a plurality of waveguides with different lengths and discretely
changing the transmission delay of the optical signals, wherein the
delay difference adjusting unit further adjusts the transmission
delay difference of the optical signals that is adjusted by the
optical buffer unit.
4. The optical signal transmission control apparatus according to
claim 1, wherein the delay difference adjusting unit adjusts the
transmission delay difference between a plurality of optical
signals, which are transmitted via redundant routes and multiplexed
using wavelength division multiplexing, by signals with same
wavelength.
5. The optical signal transmission control apparatus according to
claim 4, wherein the delay difference adjusting unit receives the
optical signal on which a delay difference detecting signal for
detecting the transmission delay difference of the optical signal
is multiplexed, and adjusts the transmission delay difference
between the optical signals by referring to the delay difference
detecting signal.
6. The optical signal transmission control apparatus according to
claim 1, further comprising a signal interruption detecting unit
that detects generation of a signal interruption; an optical signal
output unit that combines and outputs the optical signals of
respective routes; and a signal level adjusting unit that, when the
signal interruption is detected by the signal interruption
detecting unit, adjusts an intensity level of the optical signal at
each route in which the degradation of the waveform is compensated
by the waveform degradation compensating unit so that the intensity
level of the optical signal output from the optical signal output
unit is of a predetermined level.
7. The optical signal transmission control apparatus according to
claim 6, further comprising an optical buffer unit that adjusts the
transmission delay difference of the optical signals, by switching
the waveguides with different lengths and discretely changing the
transmission delay of the optical signal, wherein the delay
difference adjusting unit further adjusts the delay difference of
the optical signals in which the transmission delay difference is
adjusted by the optical buffer unit; and the signal interruption
detecting unit detects the signal interruption that occurs to the
signal before the signal is input to the optical buffer unit.
8. The optical signal transmission control apparatus according to
claim 6, wherein the signal level adjusting unit, when the signal
interruption is detected by the signal interruption detecting unit,
performs an adjustment of the intensity levels of the optical
signals at respective route before the optical signal stops
reaching.
9. The optical signal transmission control apparatus according to
claim 1, further comprising an optical signal output unit that
combines and outputs the optical signals at respective routes; and
a signal level adjusting unit that monitors the intensity level of
the optical signal output from the optical signal output unit and
adjusts the intensity level of the optical signal at each route in
which the degradation of the waveform of the optical signal is
compensated by the waveform degradation compensating unit so that
the intensity level is of a predetermined level.
10. The optical signal transmission control apparatus according to
claim 1, further comprising a signal storage unit that electrically
stores therein the optical signals at the routes in which the
degradation of the waveform is compensated by the waveform
degradation compensating unit, and a signal reading unit that reads
out the signal at the route without signal interruption, from the
signal storage unit.
11. The optical signal transmission control apparatus according to
claim 10, wherein the delay difference adjusting unit readjusts the
transmission delay difference of the optical signal, when the
signal interruption occurred at the route is absorbed, and the
signal at the route in which the signal interruption is absorbed is
read out from the signal storage unit by the signal reading
unit.
12. The optical signal transmission control apparatus according to
claim 1, further comprising an error signal detecting unit that
detects a communication error signal generated when the delay
difference exists in the optical signals at the routes, wherein the
delay difference adjusting unit adjusts the transmission delay
difference between the optical signals at each route, until the
error signal detecting unit detects no communication error
signal.
13. An optical signal transmission control method that controls
transmission of optical signals transmitted via a plurality of
redundant routes, the optical signal transmission control method
comprising: delay difference adjusting for adjusting a transmission
delay difference between the optical signals of each route by
converting a wavelength of the optical signal and making the
optical signal with a converted wavelength pass through a waveguide
in which a transmission delay of the optical signal changes
continuously depending on the wavelength; and waveform degradation
compensating for compensating degradation of a waveform of the
optical signal, while maintaining the transmission delay difference
adjusted in the delay difference adjusting.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical signal
transmission control apparatus and an optical signal transmission
control method that control transmission of optical signals
transmitted via a plurality of redundant routes. More particularly,
the present invention relates to an optical signal transmission
control apparatus and an optical signal transmission control method
that can continuously adjust a transmission delay difference
between optical signals transmitted through a plurality of
redundant routes, while preventing degradation of an optical
signal.
[0003] 2. Description of the Related Art
[0004] In the past, in optical communications, an introduction of a
wavelength division multiplexing (WDM) technology has considerably
expanded transmission capacities of optical fiber lines. In recent
years, to provide against failure of the optical fiber line, a
method that provides active and reserved redundant optical fiber
lines, and switches to the reserved optical fiber line when a
failure occurs in the active optical fiber line has been
adopted.
[0005] Communication interruption time caused when switching from
the active to the reserved optical fiber lines is generally defined
to be equal to or less than 50 milliseconds. To meet the need of
high quality communications that does not allow a communication
interruption of data even for one bit, a hitless protection
switching technique is used.
[0006] In the hitless protection switching technique, optical
signals received via active and reserved optical fiber lines are
converted into electric signals, and data converted into the
electric signals is temporarily stored in a memory. When a failure
occurs in the active optical fiber line, a switching process is
performed so that the data of the reserved-system is read out from
the memory, instead of the data of the active-system.
[0007] In this manner, when data is temporarily stored in a memory,
communication can be continued by switching the optical fiber line
to be used from the active system to the reserved system, without
causing a data error even for one bit, even if a failure occurs in
an active optical fiber line. At the same time, a transmission
delay difference of data generated between the active optical fiber
line and the reserved optical fiber line can be absorbed.
[0008] However, there are some problems to the hitless protection
switching technique using a memory. More particularly, the storage
capacity required for a memory increases drastically in proportion
to a product of a maximum value of a transmission delay difference
of data and a communication speed. When optical signals with
different wavelengths are multiplexed for transmission as in the
WDM, a memory needs to be provided for each wavelength. Further, in
the hitless protection switching technique, an optical signal needs
to be converted into an electric signal, whereby a problem is
caused that the scale of the configuration increases along the
increase of the number of wavelengths.
[0009] Therefore, a development of a technology that performs
hitless protection switching using an optical signal as it is has
been sought after. However, unlike electric signals, it is
difficult to keep optical signals to one place. Therefore, it is
difficult to store information in the form of optical signals.
Further, because the optical signal has a high propagation speed,
even if the transmission delay difference of data generated between
the active system and the reserved system is minute, a transmission
delay needs to be generated by making an optical signal pass
through a very long optical fiber in order to adjust the delay.
[0010] For example, a difference in distance between the routes of
the active system and the reserved system is generally considered
to be equal to or more than 600 kilometers. However, to adjust the
transmission delay difference generated by the difference in
distance, a waveguide of an optical signal that is equal to or more
than 600 kilometers is required. In this case, if a waveguide of
600 kilometers is formed by connecting a plurality of optical
fibers of 30 kilometers in length, a transmission delay difference
that corresponds to the maximum of 15 kilometers cannot be
adjusted.
[0011] If the difference of distance is 600 kilometers, the
transmission delay time difference between the active system and
the reserved system is approximately 3 milliseconds. However, in
addition to the transmission delay time difference caused by the
difference in distance, a transmission delay time difference is
caused by a change in temperature environment where the optical
fiber line is being laid. More particularly, the transmission delay
time difference is caused by an expansion and a contraction of an
optical fiber due to a change in the temperature environment, and
is approximately 50 nanoseconds per 100 kilometers (approximately
300 nanoseconds to 600 kilometers).
[0012] To adjust the transmission delay time difference generated
by a change in the temperature environment, a technology that
continuously controls the delay difference of the optical signals
is required. As such, a technology that combines an optical
wavelength converting circuit and a high-dispersion optical fiber,
and adds to the optical signal a delay that corresponds to the
wavelength converted by the optical wavelength converting circuit
is known (for example, refer to Japanese Patent Application
Laid-Open No. H8-146479).
[0013] Though the conventional technology mentioned above is
designed to continuously control the transmission delay difference
of the optical signals, it has a problem that the delayed optical
signal cannot be used for hitless protection switching as it
is.
[0014] More particularly, though the transmission of the optical
signal through a high-dispersion optical fiber can generates the
delay of the optical signal to eliminate a transmission delay
difference between the active system and the reserved system, a
waveform of the optical signal is distorted due to wavelength
dispersion. This leads to a problem that the hitless protection
switching cannot be performed while preventing degradation of the
optical signal.
[0015] This wavelength dispersion can be compensated by using a
high-dispersion optical fiber that has reverse characteristics to
the optical dispersion characteristics of the high-dispersion
optical fiber. However, in this case, the absorbed transmission
delay difference of the optical signal is generated again, thereby
making it unsuitable for the hitless protection switching.
[0016] Therefore, a development of a technology that can
continuously adjust a transmission delay difference between optical
signals that are transmitted via active and reserved optical fiber
lines, while preventing degradation of the optical signal, has
become an important issue.
SUMMARY OF THE INVENTION
[0017] It is an object of the present invention is to at least
solve the problems in the conventional technology.
[0018] To solve the problems as described above and to achieve an
object, an optical signal transmission control apparatus according
to one exemplary embodiment of the present invention is an optical
signal transmission control apparatus that controls transmission of
optical signals transmitted via a plurality of redundant routes,
and includes a delay difference adjusting unit that adjusts a
transmission delay difference between the optical signals of each
route by converting a wavelength of the optical signal and making
the optical signal with a converted wavelength pass through a
waveguide in which a transmission delay of the optical signal
changes continuously depending on the wavelength, and a waveform
degradation compensating unit that compensates degradation of a
waveform of the optical signal, while maintaining the transmission
delay difference adjusted by the delay difference adjusting
unit.
[0019] The above and other objects, features, advantages and
technical and industrial significance of this invention will be
better understood by reading the following detailed description of
presently preferred embodiments of the invention, when considered
in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a functional configuration diagram of a hitless
protection switching system according to a first embodiment of the
present invention;
[0021] FIG. 2 is a functional configuration diagram of a
low-dispersion fiber buffer shown in FIG. 1;
[0022] FIG. 3 is a functional configuration diagram of a
multi-wavelength light variable delay unit shown in FIG. 2;
[0023] FIG. 4 is a functional configuration diagram of a one-step
wavelength converter shown in FIG. 3;
[0024] FIG. 5 is a graph showing an example of a dispersion
characteristic curve of a dispersion compensation fiber shown in
FIG. 3;
[0025] FIG. 6 is a functional configuration diagram of a two-step
wavelength converter shown in FIG. 3;
[0026] FIG. 7 is a first explanatory diagram of a conversion
process performed by the two-step wavelength converters to convert
wavelengths of multiplexed optical signals in bulk in two
steps;
[0027] FIG. 8 is a second explanatory diagram of a conversion
process performed by the two-step wavelength converters to convert
wavelengths of multiplexed optical signals in bulk in two
steps;
[0028] FIG. 9 is an explanatory diagram of functions of dispersion
compensation fibers;
[0029] FIG. 10 is an explanatory diagram of a wavelength conversion
process at the multi-wavelength light variable delay unit;
[0030] FIG. 11 is an explanatory diagram of a delay time difference
absorbing process performed by the multi-wavelength light variable
delay unit;
[0031] FIG. 12 is an explanatory diagram of a modification of a
delay difference absorbing process performed by the
multi-wavelength light variable delay units;
[0032] FIG. 13 is an explanatory diagram of a signal-waveform
distortion correction process performed by delay amount constant
dispersion compensating units;
[0033] FIG. 14 is a functional configuration diagram of the delay
amount constant dispersion compensating unit using a fiber Bragg
grating dispersion compensation fiber;
[0034] FIG. 15 is an explanatory diagram of dispersion compensation
with a fiber Bragg grating;
[0035] FIG. 16 is an explanatory diagram of a first method to
adjust an intensity level of a signal;
[0036] FIG. 17 is an explanatory diagram of a second method to
adjust an intensity level of a signal;
[0037] FIG. 18 is a functional configuration diagram of a
controller shown in FIG. 1;
[0038] FIG. 19 is a flowchart showing a procedure of a startup
process performed by a hitless protection switching apparatus shown
in FIG. 1;
[0039] FIG. 20 is a flowchart showing a procedure of a signal
intensity level adjustment process when a signal interruption
occurs;
[0040] FIG. 21 is a flowchart showing a procedure of a recovery
process performed by the hitless protection switching apparatus of
FIG. 1, when recovering from a signal interruption;
[0041] FIG. 22 is a functional configuration diagram of a hitless
protection switching system that adjusts a delay amount of only a
0-system signal;
[0042] FIG. 23 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes only one
dispersion compensation fiber;
[0043] FIG. 24 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes only one
two-step wavelength converter;
[0044] FIG. 25 is an explanatory diagram of a setting location of a
two-step wavelength converter;
[0045] FIG. 26 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes one
two-step wavelength converter and one dispersion compensation
fiber;
[0046] FIG. 27 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes two
two-step wavelength converters, and two dispersion compensation
fibers that have reversed dispersion slopes to each other;
[0047] FIG. 28 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes a one-step
wavelength converter, a two-step wavelength converter, and
dispersion compensation fibers that have reversed dispersion slopes
to each other;
[0048] FIG. 29 is a functional configuration diagram of a hitless
protection switching system that adjusts a delay amount between a
0-system signal and a 1-system signal by monitoring a communication
error;
[0049] FIG. 30 is a functional configuration diagram of a route
switching system that performs a route switching process using an
optical switch;
[0050] FIG. 31 is a functional configuration diagram of a hitless
protection switching system that separates and outputs respective
signals; and
[0051] FIG. 32 is a functional configuration diagram of a route
switching system that includes multi-wavelength light variable
delay units and memories.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] Exemplary embodiments of an optical signal transmission
control apparatus and an optical signal transmission control method
according to the present invention will be described in detail
below with reference to the accompanying drawings. However, the
present invention is not limited to these embodiments.
[0053] A functional configuration of a hitless protection switching
system according to a first embodiment of the present invention
will be explained first. The hitless protection switching system is
a system that continues to transmit optical signals without
interruption, even when a failure occurs in any of transmission
routes among a plurality of redundant transmission routes, by using
optical signals of the other transmission route.
[0054] More particularly, the hitless protection switching system
includes a wavelength converting unit that converts each wavelength
of multiplexed optical signals, and an optical fiber whose
transmission speed of an optical signal is wavelength dependent.
Accordingly, the transmission speed of each multiplexed optical
signal is continuously adjusted, and a transmission delay
difference between the optical signals that pass through the
redundant transmission routes is absorbed. The hitless protection
switching system also compensates waveform distortion of the
optical signal and prevents degradation of the optical signal,
while keeping the transmission delay difference between the optical
systems absorbed.
[0055] FIG. 1 is a functional configuration diagram of a hitless
protection switching system according to the first embodiment of
the present invention. As shown in FIG. 1, the hitless protection
switching system includes a wavelength division multiplexing (WDM)
transmission apparatus 10, a branching apparatus 20, a hitless
protection switching apparatus 30, and a WDM transmission apparatus
40. The branching apparatus 20 and the hitless protection switching
apparatus 30 are connected via a 0-system optical fiber line 50 and
a 1-system optical fiber line 60 that are redundant transmission
routes of optical signals with different lengths.
[0056] The WDM transmission apparatus 10 is an apparatus that
multiplexes optical signals transmitted from an optical transmitter
(not shown) by a wavelength division multiplexing (WDM) system, and
transmits a plurality of multiplexed optical signals with different
wavelengths. The WDM transmission apparatus 40 is an apparatus that
receives the multiplexed optical signals with different wavelengths
from the hitless protection switching apparatus 30, and transmits
the optical signals to an optical receiver (not shown) by
separating the optical signals by each wavelength.
[0057] The branching apparatus 20 is an apparatus that multiplexes
an optical signal for delay adjustment to the optical signals
received from the WDM transmission apparatus 10, and then branches
and transmits the resulting optical signals. The branching
apparatus 20 includes a delay adjustment light source 200, a
wavelength multiplexing unit 201, and a branching unit 202. The
delay adjustment light source 200 is a light source that generates
an optical signal with a predetermined wavelength, to which the
hitless protection switching apparatus 30 refers, in order to
adjust a delay of the optical signal that has passed through one of
the 0-system optical fiber line 50 and the 1-system optical fiber
line 60. More particularly, the delay adjustment light source 200
generates two delay adjustment signals with different
wavelengths.
[0058] The wavelength multiplexing unit 201 is a processing unit
that multiplexes the signal received from the WDM transmission
apparatus 10 and the two delay adjustment signals with different
wavelengths generated by the delay adjustment light source 200, and
transmits the resulting signals to the branching unit 202. The
branching unit 202 is a processing unit that branches the optical
signals received from the wavelength multiplexing unit 201 and
transmits via the 0-system optical fiber line 50 and the 1-system
optical fiber line 60.
[0059] The hitless protection switching apparatus 30 is an
apparatus that continues to transmit optical signals without
interruption, even when a failure occurs in one of the 0-system
optical fiber line 50 and the 1-system optical fiber line 60, by
using optical signals of the other optical fiber line.
[0060] The hitless protection switching apparatus 30 converts
wavelengths of the optical signals that have passed through the
0-system optical fiber line 50 and the 1-system optical fiber line
60 and makes the resulting optical signals pass through an optical
fiber whose transmission speed of optical signals is wavelength
dependent, thereby continuously adjusts the transmission delay
difference of the optical signals that have passed through the
0-system optical fiber line 50 and the 1-system optical fiber line
60. The hitless protection switching apparatus 30, after adjusting
the transmission delay difference of the optical signal, also
compensates waveform distortion of the optical signal while
maintaining the transmission delay difference.
[0061] The hitless protection switching apparatus 30 includes
amplifiers 300a and 300b, dispersion compensating units 301a and
301b, amplifiers 302a and 302b, low-dispersion fiber buffers 303a
and 303b, amplifiers 304a and 304b, multi-wavelength light variable
delay units 305a and 305b, delay amount constant dispersion
compensating units 306a and 306b, amplifiers 307a and 307b,
former-stage variable attenuators 308a and 308b, wavelength
separating units 309a and 309b, latter-stage variable attenuators
310a and 310b, a light interruption detecting unit 311, a level
adjusting unit 312, a multiplexer 313, and a controller 314.
[0062] Among these, the amplifiers 300a, 300b, 302a, 302b, 304a,
304b, 307a, and 307b are amplifiers that amplify an optical signal.
The dispersion compensating units 301a and 301b are processing
units that compensate wavelength dispersion of optical signals that
have passed through the 0-system optical fiber line 50 and the
1-system optical fiber line 60. The dispersion compensating units
301a and 301b are formed by a dispersion compensation fiber (DCF),
for example.
[0063] The low-dispersion fiber buffers 303a and 303b are
processing units that discretely adjust a delay amount of an
optical signal. FIG. 2 is a functional configuration diagram of the
low-dispersion fiber buffers 303a and 303b. As shown in FIG. 2, the
low-dispersion fiber buffers 303a and 303b include an optical
switch 3000, a plurality of optical fibers 3001a through 3001e, and
an optical switch 3002.
[0064] The optical switch 3000 is a switch that switches the
optical fibers 3001a through 3001e, which an optical signal passes
through, depending on a required delay amount. The optical fibers
3001a through 3001e are optical fibers that have different lengths
to each other, and a delay amount of an optical signal discretely
changes depending on the lengths of the optical fibers 3001a
through 3001e. The optical switch 3002 is a switch that operates in
conjunction with the optical switch 3000, and switches the optical
fibers 3001a through 3001e, which an optical signal passes
through.
[0065] Referring back to FIG. 1, the multi-wavelength light
variable delay units 305a and 305b are processing units that
convert the wavelengths of the optical signals that are multiplexed
by the WDM transmission apparatus 10 to different wavelengths in
bulk, adjust a delay amount of the optical signal of each
wavelength converted, and absorb a transmission delay difference of
the optical signals of the 0-system optical fiber line 50 and the
1-system optical fiber line 60.
[0066] FIG. 3 is a functional configuration diagram of the
multi-wavelength light variable delay units 305a and 305b. As shown
in FIG. 3, each of the multi-wavelength light variable delay units
305a and 305b includes one-step wavelength converters 3010 and
3012, dispersion compensation fibers 3011 and 3014, two-step
wavelength converters 3013 and 3015, and temperature controllers
3016 and 3017.
[0067] The one-step wavelength converters 3010 and 3012 are
processing units that convert the wavelengths of the multiplexed
optical signals in bulk. FIG. 4 is a functional configuration
diagram of the one-step wavelength converters 3010 and 3012. As
shown in FIG. 4, each of the one-step wavelength converters 3010
and 3012 includes a variable wavelength light source 3020, a bulk
wavelength converter 3021, an optical wavelength filter 3022, and a
temperature controller 3023.
[0068] The variable wavelength light source 3020 is a light source
that generates optical signals with various wavelengths, and
outputs the generated optical signals to the bulk wavelength
converter 3021, as excitation light. The bulk wavelength converter
3021 is a wavelength conversion element such as a periodically
poled lithium niobate (PPLN), and also a converter that converts
the wavelengths of the optical signals in bulk, depending on the
wavelength of the excitation light that is input from the variable
wavelength light source 3020.
[0069] The optical wavelength filter 3022 is a filter that removes
the excitation light from the optical signal that is output from
the bulk wavelength converter 3021. The temperature controller 3023
is a controller that controls temperature of the one-step
wavelength converters 3010 and 3012.
[0070] Referring back to FIG. 3, the dispersion compensation fibers
3011 and 3014 are optical fibers that have such dispersion
characteristics that the transmission speed of an optical signal
that passes through the dispersion compensation fibers 3011 and
3014 changes depending on a wavelength of the optical signal.
[0071] FIG. 5 is a graph showing an example of a dispersion
characteristic curve of the dispersion compensation fibers 3011 and
3014. As shown in FIG. 5, in the dispersion compensation fibers
3011 and 3014, a transmission delay time caused when an optical
signal passes through the dispersion compensation fibers 3011 and
3014 changes continuously, depending on the wavelength of the
optical signal. Therefore, the transmission delay time can be
controlled, by adjusting the wavelength of the optical signal.
[0072] For example, when a wavelength band range of the multiplexed
optical signals that corresponds to a region where the dispersion
characteristic curve inclines gradually, the delay difference that
occurs to each optical signal within the wavelength band becomes
small. Accordingly, the transmission delay time of the entire
optical signals of the wavelength band also becomes small. When a
wavelength band range of the multiplexed optical signals that
corresponds to a region where the dispersion characteristic curve
inclines steeply, the delay difference that occurs to each optical
signal within the wavelength band becomes large. Accordingly, the
transmission delay time of the entire optical signals of the
wavelength band also becomes large.
[0073] In the above, an example in which the transmission delay
time increases as the wavelength increase is explained. However,
the dispersion compensation fibers 3011 and 3014 may have a
reversed dispersion characteristic curve, that is, the dispersion
compensation fibers 3011 and 3014 in which the transmission delay
time decreases as the wavelength increase may be used.
[0074] Referring back to FIG. 3, the two-step wavelength converters
3013 and 3015 are converters that convert the wavelengths of
multiplexed optical signals in bulk in two steps. FIG. 6 is a
functional configuration diagram of the two-step wavelength
converters 3013 and 3015. As shown in FIG. 6, each of the two-step
wavelength converters 3013 and 3015 includes variable wavelength
light sources 3030 and 3033, bulk wavelength converters 3031 and
3034, optical wavelength filters 3032 and 3035, and a temperature
controller 3036.
[0075] The variable wavelength light sources 3030 and 3033 are
light sources that generate optical signals of various wavelengths,
and respectively output the generated optical signals to the bulk
wavelength converters 3031 and 3034, as excitation light. The bulk
wavelength converters 3031 and 3034 are wavelength conversion
elements such as a PPLN, and also converters that convert the
wavelengths of the optical signals in bulk, depending on the
wavelength of the excitation light that is input by the variable
wavelength light sources 3030 and 3033.
[0076] The optical wavelength filters 3032 and 3035 are filters
that remove excitation light from the optical signals output from
the bulk wavelength converters 3031 and 3034. The temperature
controller 3036 is a controller that controls temperature of the
two-step wavelength converters 3013 and 3015.
[0077] FIGS. 7 and 8 are first and second explanatory diagrams of a
conversion process performed by the two-step wavelength converters
3013 and 3015 to convert the wavelengths of the multiplexed optical
signals in bulk in two steps.
[0078] As shown in FIG. 7, the variable wavelength light source
3030 inputs excitation light to the bulk wavelength converter 3031.
With the action of the bulk wavelength converter 3031, the
wavelengths of input signals Ch1, Ch2, Ch3, and Ch4 are converted,
and conversion signals Ch1, Ch2, Ch3, and Ch4 are generated. The
conversion signals Ch1, Ch2, Ch3, and Ch4 each have a wavelength
which can be represented by a symmetrical position to the
wavelength of the corresponding input signal about the twice the
wavelength of the input excitation light.
[0079] The conversion signals Ch1, Ch2, Ch3, and Ch4 respectively
correspond to the input signals Ch1, Ch2, Ch3, and Ch4. If the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 increase in
an order of Ch1, Ch2, Ch3, and Ch4, the wavelengths of the
conversion signals Ch1, Ch2, Ch3, and Ch4 decrease in this
order.
[0080] When the conversion signals Ch1, Ch2, Ch3, and Ch4 are input
into the bulk wavelength converter 3034 as input signals, the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 are
converted by the action of the bulk wavelength converter 3034,
whereby conversion signals Ch1, Ch2, Ch3, and Ch4 are generated.
The conversion signals Ch1, Ch2, Ch3, and Ch4 each have a
wavelength which can be represented by a symmetrical position to
the wavelength of the corresponding input signal about the twice
the wavelength of the excitation light input by the variable
wavelength light source 3033.
[0081] The conversion signals Ch1, Ch2, Ch3, and Ch4 respectively
correspond to the input signals Ch1, Ch2, Ch3, and Ch4. If the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 increase in
an order of Ch4, Ch3, Ch2, and Ch1, the wavelengths of the
conversion signals Ch1, Ch2, Ch3, and Ch4 increase in this
order.
[0082] In this manner, when the two-step wavelength converters 3013
and 3015 are used, the wavelength can be shifted towards a larger
side, only by a difference between the wavelength of the input
signal Ch1 and the wavelength of the conversion signal Ch1. If the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 are
increased in an order of Ch1, Ch2, Ch3, and Ch4, the wavelengths of
the conversion signals Ch1, Ch2, Ch3, and Ch4 that respectively
correspond to the input signals Ch1, Ch2, Ch3, and Ch4, can be
increased in an order of Ch1, Ch2, Ch3, and Ch4.
[0083] In each of the one-step wavelength converters 3010 and 3012
shown in FIG. 4, the wavelength is converted only once.
Accordingly, if the wavelengths of the input signals Ch1, Ch2, Ch3,
and Ch4 are increased in an order of Ch1, Ch2, Ch3, and Ch4, the
wavelengths of the conversion signals Ch1, Ch2, Ch3, and Ch4 that
respectively correspond to the input signals Ch1, Ch2, Ch3, and
Ch4, are increased in an order of Ch4, Ch3, Ch2, and Ch1.
[0084] As shown in FIG. 8, the wavelength of the optical signal can
be converted so as to decrease, by adjusting the wavelength of the
excitation light. When the variable wavelength light source 3030
inputs excitation light to the bulk wavelength converter 3031, with
the action of the bulk wavelength converter 3031, the wavelengths
of input signals Ch1, Ch2, Ch3, and Ch4 are converted, and
conversion signals Ch1, Ch2, Ch3, and Ch4 are generated. The
conversion signals Ch1, Ch2, Ch3, and Ch4 each have a wavelength
which can be represented by a symmetrical position to the
wavelength of the corresponding input signal about the twice the
wavelength of the input excitation light.
[0085] The conversion signals Ch1, Ch2, Ch3, and Ch4 respectively
correspond to the input signals Ch1, Ch2, Ch3, and Ch4. If the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 increase in
an order of Ch1, Ch2, Ch3, and Ch4, the wavelengths of the
conversion signals Ch1, Ch2, Ch3, and Ch4 decrease in this
order.
[0086] When the conversion signals Ch1, Ch2, Ch3, and Ch4 are input
into the bulk wavelength converter 3034 as input signals, the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 are
converted by the action of the bulk wavelength converter 3034,
whereby conversion signals Ch1, Ch2, Ch3, and Ch4 are generated.
The conversion signals Ch1, Ch2, Ch3, and Ch4 each have a
wavelength which can be represented by a symmetrical position to
the wavelength of the corresponding input signal about the twice
the wavelength of the excitation light input by the variable
wavelength light source 3033.
[0087] The conversion signals Ch1, Ch2, Ch3, and Ch4 respectively
correspond to the input signals Ch1, Ch2, Ch3, and Ch4. If the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 increase in
an order of Ch4, Ch3, Ch2, and Ch1, the wavelengths of the
conversion signals Ch1, Ch2, Ch3, and Ch4 increase in this
order.
[0088] In this manner, when the two-step wavelength converters 3013
and 3015 are used, the wavelength can be shifted towards a smaller
side, only by a difference between the wavelength of the input
signal Ch1 and the wavelength of the conversion signal Ch1. If the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4 are
increased in an order of Ch1, Ch2, Ch3, and Ch4, the wavelengths of
the conversion signals Ch1, Ch2, Ch3, and Ch4 that respectively
correspond to the input signals Ch1, Ch2, Ch3, and Ch4, can be
increased in an order of Ch1, Ch2, Ch3, and Ch4.
[0089] In each of the one-step wavelength converters 3010 and 3012
shown in FIG. 4, the wavelength is converted only once.
Accordingly, if the wavelengths of the input signals Ch1, Ch2, Ch3,
and Ch4 are increased in an order of Ch1, Ch2, Ch3, and Ch4, the
wavelengths of the conversion signals Ch1, Ch2, Ch3, and Ch4 that
respectively correspond to the input signals Ch1, Ch2, Ch3, and
Ch4, are increased in an order of Ch4, Ch3, Ch2, and Ch1.
[0090] As explained in FIGS. 7 and 8, the delay time difference
between the optical signals can be adjusted by converting the
wavelengths of the optical signals, and by making the optical
signals with a converted wavelength pass through the dispersion
compensation fibers 3011 and 3014, and the two-step wavelength
converters 3013 and 3015 shown in FIG. 3.
[0091] This will be explained with reference to FIGS. 9 through 12.
FIG. 9 is an explanatory diagram of functions of dispersion
compensation fibers 3041 and 3043. The dispersion compensation
fibers 3041 and 3043 are considered to have the dispersion
characteristics shown in FIG. 5.
[0092] As shown in FIG. 9, the input signals are converted into the
conversion signals by using a one-step wavelength converter 3040
similar to the one shown in FIG. 4. In the input signals, the
wavelengths increase in an order of Ch1, Ch2, Ch3, and Ch4.
However, in the conversion signals, the wavelengths increase in an
order of Ch4, Ch3, Ch2, and Ch1. If the conversion signals are made
to pass through the dispersion compensation fiber 3041 that has the
dispersion characteristics as shown in FIG. 5, the delay time
increases as the wavelength of the input signal increases.
[0093] Next, the signals that have passed through the dispersion
compensation fiber 3041 are considered to be input signals, and the
input signals are converted into the conversion signals, using a
one-step wavelength converter 3042 that is similar to the one shown
in FIG. 4. In the input signals, the wavelengths increase in an
order of Ch4, Ch3, Ch2, and Ch1. However, in the conversion
signals, the wavelengths increase in an order of Ch1, Ch2, Ch3, and
Ch4. When the conversion signals are made to pass through the
dispersion compensation fiber 3043 that has the dispersion
characteristics as shown in FIG. 5, the delay time difference
between the respective signals Ch1, Ch2, Ch3, and Ch4 can be
compensated. This is because the delay time increases as the
wavelength of the input signal increases.
[0094] The same applies for the multi-wavelength light variable
delay units 305a and 305b shown in FIG. 3. FIG. 10 is an
explanatory diagram of a wavelength conversion process at the
multi-wavelength light variable delay units 305a and 305b.
[0095] As shown in FIG. 10, the input signals Ch1, Ch2, Ch3, and
Ch4 are considered to be input into the one-step wavelength
converter 3010 of the multi-wavelength light variable delay units
305a and 305b. The wavelengths of the input signals Ch1, Ch2, Ch3,
and Ch4 are considered to increase in this order. When the one-step
wavelength converter 3010 converts the wavelengths of the input
signals Ch1, Ch2, Ch3, and Ch4, the wavelengths of the conversion
signals Ch1, Ch2, Ch3, and Ch4 decrease in this order.
[0096] Then, the conversion signals Ch1, Ch2, Ch3, and Ch4 are made
to pass through the dispersion compensation fiber 3011 that has the
dispersion characteristics as shown in FIG. 5, where the delay time
increases as the wavelength of the input signal increases. The
delay time of the entire conversion signals Ch1, Ch2, Ch3, and Ch4
can be adjusted, by changing a wavelength shift amount
.DELTA..lamda..
[0097] The one-step wavelength converter 3012 converts the
wavelengths of the conversion signals Ch1, Ch2, Ch3, and Ch4. The
magnitude relationship of the wavelengths of the conversion signals
Ch1, Ch2, Ch3, and Ch4 are reverse to the wavelengths of the input
signals Ch1, Ch2, Ch3, and Ch4. If a wavelength shift amount is
.DELTA..lamda., the wavelengths of the conversion signals Ch1, Ch2,
Ch3, and Ch4 can be made the same as the wavelengths of the input
signals Ch1, Ch2, Ch3, and Ch4 of the one-step wavelength converter
3010.
[0098] Subsequently, the two-step wavelength converter 3013
receives the conversion signals Ch1, Ch2, Ch3, and Ch4 output from
the one-step wavelength converter 3012, as input signals. The
two-step wavelength converter 3013 converts the wavelengths in two
steps, thereby generating the conversion signals Ch1, Ch2, Ch3, and
Ch4. Because the two-step wavelength converter 3013 converts the
wavelength in two steps, the magnitude relationship of the
wavelengths of the input signals Ch1, Ch2, Ch3, and Ch4, and the
magnitude relationship of the wavelengths of the conversion signals
Ch1, Ch2, Ch3, and Ch4 that are output eventually does not
change.
[0099] When the conversion signals Ch1, Ch2, Ch3, and Ch4 are made
to pass through the dispersion compensation fiber 3014 that has the
dispersion characteristics as shown in FIG. 5, the delay time
increases as the wavelength of the input signal increases.
Accordingly, the delay time among the signals Ch1, Ch2, Ch3, and
Ch4 generated at the dispersion compensation fiber 3011 can be
cancelled. Further, the delay time of the entire signals Ch1, Ch2,
Ch3, and Ch4 can be adjusted, by changing the wavelength shift
amount .DELTA..lamda..
[0100] The delay time differences among the respective signals Ch1,
Ch2, Ch3, and Ch4 are cancelled, by making the wavelength shift
amounts in the wavelength conversion in the one-step wavelength
converters 3010 and 3012, and the two-step wavelength converters
3013 and 3015 the same. Alternatively, the respective signals Ch1,
Ch2, Ch3, and Ch4 can be output in a state that the delay time
difference exists, by changing the shift amount.
[0101] In the present embodiment, the delay time difference of the
optical signals that have passed through the redundant 0-system
optical fiber line 50 and the 1-system optical fiber line 60 is
absorbed, by using the principle described with reference to FIGS.
9 and 10. FIG. 11 is an explanatory diagram of a delay time
difference absorbing process performed by the multi-wavelength
light variable delay units 305a and 305b. In FIG. 11, a
transmission route of an optical signal of one of the 0-system and
the 1-system is shown.
[0102] As shown in FIG. 11, when the respective input signals S1,
Ch1, Ch2, Ch3, Ch4, and S2 that have a maximum delay time
difference W are input into the one-step wavelength converter 3010,
the one-step wavelength converter 3010 converts the wavelengths of
the input signals S1, Ch1, Ch2, Ch3, Ch4, and S2. The dispersion
compensation fiber 3011 then generates delays of the respective
times Ta0, Ta1, Ta2, Ta3, Ta4, and Ta5, according to the
wavelengths of the respective input signals S1, Ch1, Ch2, Ch3, Ch4,
and S2.
[0103] Among the wavelengths of the respective input signals S1,
Ch1, Ch2, Ch3, Ch4, and S2, a wavelength spacing between the
adjacent wavelengths is made equal. The signals S1 and S2 are delay
adjustment signals that are generated by the delay adjustment light
source 200 of the branching apparatus 20.
[0104] The wavelengths of the signals S1, Ch1, Ch2, Ch3, Ch4, and
S2 output from the dispersion compensation fiber 3011 are converted
by the one-step wavelength converter 3012, and the resulting
signals are input into the two-step wavelength converter 3013. The
two-step wavelength converter 3013 then converts the wavelengths of
the signals S1, Ch1, Ch2, Ch3, Ch4, and S2. The dispersion
compensation fiber 3014 generates the delays of the respective
times of Tb0, Tb1, Tb2, Tb3, Tb4, and Tb5, according to the
wavelengths of the respective signals S3, Ch1, Ch2, Ch3, Ch4, and
S2.
[0105] Therefore, delay times T0, T1, T2, T3, T4, and T5 of the
respective signals S1, Ch1, Ch2, Ch3, Ch4, and 32, when the signals
S1, Ch1, Ch2, Ch3, Ch4, and S2 pass through one of the
multi-wavelength light variable delay units 305a and 305b, may be
respectively expressed as follows:
S1:T0=Ta0+Tb0,
Ch1:T1=Ta1+Tb1,
Ch2:T2=Ta2+Tb2,
Ch3:T3=Ta3+Tb3,
Ch4:T4=Ta4+Tb4,
S2:T5=Ta5+Tb5.
[0106] By changing the wavelengths of the signals S1, Ch1, Ch2,
Ch3, Ch4, and S2, the delay times Ta0, Ta1, Ta2, Ta3, Ta4, and Ta5,
and the delay times Tb0, Tb1, Tb2, Tb3, Tb4, and Tb5 can be set
variably, according to the wavelength dispersion characteristics of
the dispersion compensation fibers 3011 and 3014 shown in FIG.
5.
[0107] Therefore, when the one-step wavelength converters 3010 and
3012, and the two-step wavelength converters 3013 and 3015 convert
the wavelengths, a wavelength shift amount is determined so that
the delay times T0, T1, T2, T3, T4, and T5 are as follows:
S1:T0=Ta0+Tb0=W+D
Ch1:T1=Ta1+Tb1=4/5W+D,
Ch2:T2=Ta2+Tb2=3/5W+D,
Ch3:T3=Ta3+Tb3=2/5W+D,
Ch4:T4=Ta4+Tb4=1/5W+D,
S2:T5=Ta5+Tb5=D.
In the above, D indicates a delay time of the entire signals S1,
Ch1, Ch2, Ch3, Ch4, and S2.
[0108] Through the wavelength conversion by the one-step wavelength
converters 3010 and 3012 and the two-step wavelength converters
3013 and 3015, the delay time difference among the respective
signals S1, Ch1, Ch2, Ch3, Ch4, and S2 can be absorbed, and the
delay time of the entire signals S1, Ch1, Ch2, Ch3, Ch4, and S2 can
be adjusted to a predetermined value D. By performing such a
process at both routes of the 0-system and the 1-system, the delay
time difference of the signals of the 0-system and the 1-system can
be absorbed.
[0109] The adjustment of the delay time difference will be
explained in further detail. The dispersion characteristic curve of
the dispersion compensation fibers 3011 and 3014 shown in FIG. 5
will be indicated by F(x). Here, x indicates a wavelength.
[0110] In this case, when a signal whose wavelength is included in
a predetermined wavelength band passes through the dispersion
compensation fiber 3011, the delay time Ta is expressed as
follows:
Ta=dF(x)/dx|.sub.x=H.DELTA.x+F(H).
[0111] In the above formula, H indicates the center wavelength of
the wavelength band of each signal on which the wavelength
conversion is performed by the one-step wavelength converter 3010,
.DELTA.x indicates a difference between the wavelength H and a
wavelength of a certain signal included in the wavelength band, and
dF(x)/dx|.sub.x=H indicates an inclination of the dispersion
characteristic curve F(x) that corresponds to the wavelength H.
Therefore, dF(x)/dx|.sub.x=H.DELTA.x is a delay time difference
between a signal that has the wavelength H and a signal that has
the wavelength H+.DELTA.x. Further, F(H) indicates a delay time of
a signal that has the wavelength H.
[0112] Similarly, when the wavelength of a signal whose wavelength
is included in a predetermined wavelength band is converted, and
the resulting signal passes through the dispersion compensation
fiber 3104, a delay time Tb is expressed as follows:
Tb=dF(x)/dx|.sub.x=L.DELTA.x+F(L).
[0113] In the above formula, L indicates the center wavelength of
the wavelength band of each signal on which the wavelength
conversion is performed by the two-step wavelength converter 3013,
.DELTA.x indicates a difference between a wavelength L and a
wavelength of a certain signal included in the wavelength band, and
dF(x)/dx|.sub.x=L indicates an inclination of the dispersion
characteristic curve F(x) corresponding to the wavelength L.
Therefore, dF(x)/dx|.sub.x=L.DELTA.x is a delay time difference
between a signal that has the wavelength L and a signal that has
the wavelength L+.DELTA.x. Further, F(L) indicates a delay time of
the signal that has the wavelength L.
[0114] Accordingly, the delay time T, when the signal passes
through the dispersion compensation fiber 3011 and the dispersion
compensation fiber 3014, is expressed as follows:
T = F ( x ) / x x = H .DELTA. x + F ( H ) + F ( x ) / x x = L
.DELTA. x + F ( L ) = ( F ( x ) / x x = H + F ( x ) / x x = L )
.DELTA. x + F ( H ) + F ( L ) = ( A 1 + A 2 ) .DELTA. x + B 1 + B
2. ##EQU00001##
[0115] In the above formula,
A1=dF(x)/dx|.sub.x=H,
A2=dF(x)/dx|.sub.x=L,
B1=F(H),
B2=F(L).
[0116] As can be seen from the formula, when F(x) is a function of
x of an order equal to or higher than 2, dF(x)/dx is a function of
x of an order equal to or higher than 1. In other words, A1+A2 is a
function of H and L of an order equal to or higher than 1, and
B1+B2 is a function of H and L of an order equal to or higher than
2.
[0117] In the above formula, (A1+A2).DELTA.x indicates a delay time
difference between a signal whose wavelength was H and a signal
whose wavelength was H+.DELTA.x before passing through the
dispersion compensation fibers 3011 and 3014, after having passed
through the dispersion compensation fibers 3011 and 3014, and B1+B2
indicates a delay time of a signal whose wavelength was H before
passing through the dispersion compensation fibers 3011 and 3014,
after having passed through the dispersion compensation fibers 3011
and 3014.
[0118] By converting the wavelength of the signal and making the
signal pass through the dispersion compensation fibers 3011 and
3014, the delay time B1+B2 of the signal can be adjusted, while the
delay time difference (A1+A2).DELTA.x between the signals is kept
constant. Here, .DELTA.x indicates a constant value. Alternatively,
(A1+A2).DELTA.x can be adjusted, while B1+B2 is kept constant.
[0119] For example, when F(x)=x.sup.2,
(A1+A2).DELTA.x=2(H+L).DELTA.x,
B1+B2=H.sup.2+L.sup.2.
[0120] Therefore, by adjusting the wavelengths H and L, the delay
time of the entire signals, i.e.,
B1+B2=H.sup.2+L.sup.2=H.sup.2+(C/2/.DELTA.x-H).sup.2 can be changed
continuously, while the time delay difference 2(H+L).DELTA.x
between the signals is kept constant (i.e., 2(H+L).DELTA.x=C, where
C is a constant value). On the contrary, (A1+A2).DELTA.x can be
changed continuously, without changing the delay time of the entire
signals (i.e., (H.sup.2+L.sup.2)=C, where C is a constant
value).
[0121] Accordingly, the delay time difference among the multiplexed
signals S1, Ch1, Ch2, Ch3, Ch4, and S2 that have passed through one
of the 0-system optical fiber line 50 and the 1-system optical
fiber line 60 can be absorbed. Further, the delay time difference
between the signal that has passed through the 0-system optical
fiber line 50 and the signal that has passed trough the 1-system
optical fiber line 60 can be absorbed.
[0122] In the example shown in FIG. 11, the delay time difference
among the multiplexed signals S1, Ch1, Ch2, Ch3, Ch4, and S2 that
have passed through one of the 0-system optical fiber line 50 and
the 1-system optical fiber line 60 is absorbed. However, even if
the delay time difference exists among the signals S1, Ch1, Ch2,
Ch3, Ch4, and S2, if there is no delay time difference between the
corresponding signals (i.e., between S1s, between Ch1s, between
Ch2s, between Ch3s, between Ch4s, and between S2s) of the 0-system
signals S1, Ch1, Ch2, Ch3, Ch4, and S2, and the 1-system signals
S1, Ch1, Ch2, Ch3, Ch4, and S2, the apparatus can work sufficiently
as a hitless protection switching apparatus.
[0123] FIG. 12 is an explanatory diagram of a modification of the
delay difference absorbing process performed by the
multi-wavelength light variable delay units 305a and 305b. In this
example, the delay time difference T1 is generated between the
0-system signal S1 and the 1-system signal S1, and the delay time
difference T2 is generated between the 0-system signal S2 and the
1-system signal S2. Further, a delay time difference W1 is
generated in the 0-system signals S1, Ch1, Ch2, Ch3, Ch4, and S2,
and a delay time difference W2 is generated in the 1-system signals
S1, Ch1, Ch2, Ch3, Ch4, and S2.
[0124] Even in such case, as explained in FIG. 11, through the
wavelength conversion by the one-step wavelength converters 3010
and 3012 and the two-step wavelength converters 3013 and 3015, the
delay time difference between the corresponding signals (between
S1s, between Ch1s, between Ch2s, between Ch3s, between Ch4s, and
between S2s) of the 0-system signals S1, Ch1, Ch2, Ch3, Ch4, and
S2, and the 1-system signals S1, Ch1, Ch2, Ch3, Ch4, and S2 can be
absorbed.
[0125] However, in this case, the absorption of the delay time
difference in the 0-system signals S1, Ch1, Ch2, Ch3, Ch4, and S2,
and the absorption of the delay time difference in the 1-system
signals S1, Ch1, Ch2, Ch3, Ch4, and S2 are not performed.
[0126] The following may be performed to absorb the delay
difference W1 in the 0-system signals, the delay difference W2 in
the 1-system signals, and the delay difference T1 between the
0-system signal and the 1-system signal. As shown in FIG. 12, the
delay times of the respective input signals S1, Ch1, Ch2, Ch3, Ch4,
and S2 that are generated at a 0-system dispersion compensation
fiber 3011a are made to be Ta0, Ta1, Ta2, Ta3, Ta4, and Ta5,
respectively. The delay times of the respective input signals S1,
Ch1, Ch2, Ch3, Ch4, and S2 that are generated at a 0-system
dispersion compensation fiber 3014a are made to be Tb0, Tb1, Tb2,
Tb3, Tb4, and Tb5, respectively.
[0127] The wavelength spacing between the adjacent wavelengths of
the respective input signals S1, Ch1, Ch2, Ch3, Ch4, and S2 are
made equal. The signals S1 and S2 are delay adjustment signals that
are generated by the delay adjustment light source 200 of the
branching apparatus 20.
[0128] The delay times of the respective input signals S1, Ch1,
Ch2, Ch3, Ch4, and S2 generated at a 1-system dispersion
compensation fiber 3011b are made to be Tc0, Tc1, Tc2, Tc3, Tc4,
and Tc5, respectively, and the delay times of the respective input
signals S1, Ch1, Ch2, Ch3, Ch4, and S2 generated at a 1-system
dispersion compensation fiber 3014b are made to be Td0; Td1, Td2,
Td3, Td4, and Td5, respectively.
[0129] Accordingly, the delay times of the respective input signals
S1, Ch1, Ch2, Ch3, Ch4, and S2 generated at the 0-system dispersion
compensation fibers 3011a and 3014a are respectively expressed as
follows:
S1:T0=Ta0+Tb0,
Ch1:T1=Ta1+Tb1,
Ch2:T2=Ta2+Tb2,
Ch3:T3=Ta3+Tb3,
Ch4:T4=Ta4+Tb4,
S2:T5=Ta5+Tb5.
[0130] The delay times of the respective input signals S1, Ch1,
Ch2, Ch3, Ch4, and S2 generated at the 1-system dispersion
compensation fibers 3011b and 3014b are respectively expressed as
follows:
S1:T0=Tc0+Td0,
Ch1:T1=Tc1+Td1,
Ch2:T2=Tc2+Td2,
Ch3:T3=Tc3+Td3,
Ch4:T4=Tc4+Td4,
S2:T5=Tc5+Td5.
[0131] The delay times Ta0, Ta1, Ta2, Ta3, Ta4, and Ta5, the delay
times Tb0, Tb1, Tb2, Tb3, Tb4, and Tb5, the delay times Tc0, Tc1,
Tc2, Tc3, Tc4, and Tc5, and the delay times Td0, Td1, Td2, Td3,
Td4, and Td5 can be variably set through the changes in the
wavelengths of the signals S1, Ch1, Ch2, Ch3, Ch4, and S2 according
to the wavelength dispersion characteristics of the dispersion
compensation fibers 3011a, 3011b, 3014a, and 3014b as shown in FIG.
5.
[0132] Therefore, one-step wavelength converters 3010a and 3012a,
and two-step wavelength converters 3013a and 3015a of the 0-system,
when converting wavelengths, determine a wavelength shift amount so
that the delay times T0, T1, T2, T3, T4, and T5 are as follows:
S1:T0=Ta0+Tb0=W1+D1,
Ch1:T1=Ta1+Tb1=4/5W1+D1,
Ch2:T2=Ta2+Tb2=3/5W1+D1,
Ch3:T3=Ta3+Tb3=2/5W1+D1,
Ch4:T4=Ta4+Tb4=1/5W1+D1,
S2:T5=Ta5+Tb5=D1.
Here, D1 indicates a delay amount of the entire signals S1, Ch1,
Ch2, Ch3, Ch4, and S2. This enables to absorb the delay difference
in the 0-system signals (in other words, V=0 at an output
signal).
[0133] One-step wavelength converters 3010b and 3012b, and two-step
wavelength converters 3013b and 3015b of the 1-system, when
converting wavelengths, determine a wavelength shift amount, so
that the delay times T0, T1, T2, T3, T4, and T5 are as follows:
S1:T0=Tc0+Td0=W2+D2,
Ch1:T1=Tc1+Td1=4/5W2+D2,
Ch2:T2=Tc2+Td2=3/5W2+D2,
Ch3:T3=Tc3+Td3=2/5W2+D2,
Ch4:T4=Tc4+Td4=1/5W2+D2,
S2:T5=Tc5+Td5=D2.
Here, D2 indicates a delay amount of the entire signals S1, Ch1,
Ch2, Ch3, Ch4, and S2. This enables to absorb the delay difference
in the 1-system signals (in other words, V=0 at an output
signal).
[0134] The delay difference between the 0-system signal and the
1-system signal can also be absorbed, by determining a wavelength
shift amount so that a following formula is satisfied:
D2=W1+D1-W2-T1.
[0135] Referring back to FIG. 3, the temperature controllers 3016
and 3017 are controllers that control temperatures of the one-step
wavelength converters 3010 and 3012, the dispersion compensation
fibers 3011 and 3014, and the two-step wavelength converters 3013
and 3015.
[0136] Referring back to FIG. 1, the delay amount constant
dispersion compensating units 306a and 306b are compensating units
that compensate signal waveform distortion, without changing a
transmission delay amount of a signal. More particularly, the delay
amount constant dispersion compensating units 306a and 306b
compensate signal waveform distortion, by using a fiber Bragg
grating (FBG) dispersion compensation fiber and a Virtually Imaged
Phased Array (VIPA) type dispersion compensating unit.
[0137] FIG. 13 is an explanatory diagram of a signal-waveform
distortion correction process performed by the delay amount
constant dispersion compensating units 306a and 306b. In the
present embodiment, the multi-wavelength light variable delay units
305a and 305b convert a wavelength of a signal, and adjust a delay
amount of the signal by using the wavelength dispersion
characteristics of the dispersion compensation fibers 3011 and
3014. Therefore, the signal waveform distortion increases according
to a delay adjustment amount.
[0138] As shown in FIG. 13, when the signal waveform distortion is
corrected simply by the dispersion compensation fiber 3043, the
delay adjustment amount between the signals are cancelled, and the
delay time difference is generated once again. The delay time
difference generated at this point changes depending on the length
of the dispersion compensation fiber 3043.
[0139] Therefore, in the present embodiment, the delay amount
constant dispersion compensating units 306a and 306b compensate the
signal waveform distortion, without changing the delay amount of
the signal. FIG. 14 is a functional configuration diagram of the
delay amount constant dispersion compensating units 306a and 306b
using a fiber Bragg grating dispersion compensation fiber.
[0140] As shown in FIG. 14, each of the delay amount constant
dispersion compensating units 306a and 306b includes a wavelength
separating unit 3050, optical circulators 3051a through 3051f,
variable fiber grating filters 3052a through 3052f, and a
wavelength multiplexing unit 3053.
[0141] The wavelength separating unit 3050 is a processing unit
that separates a plurality of multiplexed signals with different
wavelengths by each wavelength. The optical circulators 3051a
through 3051f are processing units that output the signals
separated for each wavelength to the variable fiber grating filters
3052a through 3052f, that are connected to the optical circulators
3051a through 3051f, respectively. The optical circulators 3051a
through 3051f also output the signals respectively output from the
variable fiber grating filters 3052a through 3052f, to the
wavelength multiplexing unit 3053.
[0142] The variable fiber grating filters 3052a through 3052f are
processing units that compensate dispersion of a signal, using the
fiber Bragg grating that can vary dispersion characteristics, by
changing a bend, a stress, and an environmental temperature, for
example.
[0143] FIG. 15 is an explanatory diagram of dispersion compensation
using a fiber Bragg grating. As shown in FIG. 15, the fiber Bragg
grating reflects a signal with a specified wavelength, by changing
a refractive index of a waveguide at a specified cycle. The
wavelength dispersion can be compensated, by changing the cycle
step-by-step, and shifting a reflection point where the signal is
reflected depending on the wavelength.
[0144] The correction amount of the wavelength dispersion is
.DELTA.T/.DELTA..lamda.(ps/nm). Here, .DELTA.T is a delay time
difference between a reflection light 1 and a reflection light 3
that can be obtained by dividing a distance .DELTA.L by a group
velocity Vg of a signal. The distance .DELTA.L is a distance
between a reflection point of the reflection light 1 and a
reflection point of the reflection light 3. Further, .DELTA..lamda.
is a difference between a wavelength of the reflection light 1 and
a wavelength of the reflection light 3.
[0145] By using the variable fiber grating filters 3052a through
3052f as shown in FIG. 15, the signal waveform distortion of each
signal that is separated by the wavelength separating unit 3050 can
be compensated.
[0146] Referring back to FIG. 14, the wavelength multiplexing unit
3053 is a processing unit that receives signals with different
wavelengths whose signal waveform distortion is corrected, from the
optical circulators 3051a through 3051f, and multiplexes each
signal.
[0147] By using such delay amount constant dispersion compensating
units 306a and 306b, the delay amount of each signal can be made
constant, by adjusting the optical circulators 3051a through 3051f
and the variable fiber grating filters 3052a through 5052f so that
the time of an output of a signal from the wavelength separating
unit 3050 until the time of an input into the wavelength
multiplexing unit 3053 is the same for each signal.
[0148] The VIPA type dispersion compensating unit can also be used,
instead of the fiber Bragg grating dispersion compensation fiber,
to compensate the signal waveform distortion without changing the
delay amount of the signal. In this case, a curved mirror is used
to reflect the signal. The signal waveform distortion is corrected
according to the shape of the curved mirror.
[0149] Referring back to FIG. 1, the former-stage variable
attenuators 308a and 308b are processing units that adjust an
intensity level of a signal. More particularly, the former-stage
variable attenuators 308a and 308b adjust the intensity levels of
the signals of the 0-system and the 1-system so as to be the same.
However, the adjustment of the intensity level is not performed
during a signal interruption.
[0150] The wavelength separating units 309a and 309b are separating
units that separate two delay adjustment signals with different
wavelengths included in each of the 0-system signal and the
1-system signal, and transmit the separated signal to the
controller 314.
[0151] The latter-stage variable attenuators 310a and 310b are
processing units that adjust an intensity level of a signal,
similarly to the former-stage variable attenuators 308a and 308b.
The latter-stage variable attenuators 310a and 310b adjust the
intensity levels of the signals of the 0-system and the 1-system,
so as to eliminate a fluctuation of the intensity level of the
signal output from the multiplexer 313, when a signal interruption
occurs in one of the 0-system optical fiber line 50 and the
1-system optical fiber line 60.
[0152] There are several ways to adjust an intensity level of a
signal. FIG. 16 is an explanatory diagram of a first method to
adjust an intensity level of a signal. A case where a signal
interruption occurs in the 1-system will be explained here.
[0153] As shown in FIG. 16, in the first method, when a signal
interruption is detected by the light interruption detecting unit
311, the latter-stage variable attenuator 310b of the 1-system,
where the signal interruption occurs, attenuates an output level of
the signal before the signal is being interrupted. Then, the
0-system latter-stage variable attenuator 310a increases the output
level of the signal.
[0154] A signal route of the hitless protection switching apparatus
30 is designed so that the time from when the light interruption
detecting unit 311 detects a signal interruption until when an
adjustment of an output level of the signal is finished by the
latter-stage variable attenuators 310a and 310b, is shorter than
the time from when the light interruption detecting unit 311
detects a signal interruption until when the signal interruption
occurs in the latter-stage variable attenuator 310b of the 1-system
where the signal interruption occurs.
[0155] In this manner, the intensity level of the signal is
adjusted before the signal interruption occurs at the latter-stage
variable attenuator 310b of the system where the signal
interruption occurs so that the signal output from the multiplexer
313 is at a constant intensity level. Therefore, the transmission
of optical signals can be continued without interruption, even if a
signal interruption occurs.
[0156] FIG. 17 is an explanatory diagram of a second method to
adjust an intensity level of a signal. Again, a case where the
signal interruption occurs in the 1-system will be explained. As
shown in FIG. 17, in the second method, when the intensity level of
the output signal of the multiplexer 313 lowers by a predetermined
amount (for example, 3 dB), the latter-stage variable attenuator
310a in the 0-system without a signal interruption increases the
output level of the signal until the intensity level of the output
signal of the multiplexer 313 returns to the level before the
signal interruption.
[0157] In this manner, when the intensity level of the output
signal of the multiplexer 313 lowers by a predetermined amount, the
intensity level of the signal is adjusted so that the intensity
level of the signal output from the multiplexer 313 is at the same
level as the intensity level before the signal interruption.
Accordingly, even if a signal interruption occurs, the transmission
of optical signals can be continued without interruption.
[0158] Referring back to FIG. 1, the light interruption detecting
unit 311 is a processing unit that detects whether a signal
interruption occurs in one of the 0-system optical fiber line 50
and the 1-system optical fiber line 60 by monitoring the signals
input into the amplifiers 300a and 300b. On detecting the signal
interruption, the light interruption detecting unit 311 notifies
the level adjusting unit 312 of the occurrence of signal
interruption.
[0159] The level adjusting unit 312 is an adjusting unit that
adjusts an output level of the signal at the latter-stage variable
attenuators 310a and 310b, when receiving a notification of signal
interruption from the light interruption detecting unit 311, as
explained in FIG. 16. The level adjusting unit 312 also adjusts an
output level of the signal at the latter-stage variable attenuators
310a and 310b, when receiving a notification that an intensity
level of the signal is lowered by a predetermined amount, from the
multiplexer 313, as explained in FIG. 17.
[0160] The multiplexer 313 is an optical coupler that combines a
0-system signal and a 1-system signal. The multiplexer 313 performs
a process of detecting whether an intensity level of a signal after
being multiplexed is lowered by a predetermined amount or more. If
the intensity level is lowered by the predetermined amount or more,
the multiplexer 313 notifies the level adjusting unit 312.
[0161] The controller 314 is a controller that controls the
one-step wavelength converters 3010 and 3012, the two-step
wavelength converters 3013 and 3015, the delay amount constant
dispersion compensating units 306a and 306b, the former-stage
variable attenuators 308a and 308b, and the latter-stage variable
attenuators 310a and 310b shown in FIG. 3 to adjust a delay
difference between the 0-system signal and the 1-system signal, a
signal waveform distortion, and an intensity level of a signal.
[0162] FIG. 18 is a functional configuration diagram of the
controller 314. As shown in FIG. 18, the controller 314 includes
interface units 3060, 3066, 3067, 3068, 3069, 3070, and 3071, a
delay adjustment value calculating unit 3061, an intensity level
adjustment value calculating unit 3062, a DCF characteristics data
storage unit 3063, a low-dispersion fiber buffer characteristics
data storage unit 3064, and a setting information generating unit
3065.
[0163] The interface unit 3060 is an interface that receives two
delay adjustment signals with different wavelengths included in
each of the 0-system signal and the 1-system signal and separated
by corresponding one of the wavelength separating units 309a and
309b.
[0164] The delay adjustment value calculating unit 3061 is a
calculating unit that detects delay differences in the respective
0-system signals, delay differences in the respective 1-system
signals, a delay difference between the 0-system signal and the
1-system signal, and the like to calculate target values for
adjusting these delay differences. More particularly, the delay
adjustment value calculating unit 3061 calculates a target value
for adjusting the delay amounts of the entire 0-system and the
1-system signals, and a target value for adjusting the delay
differences between the 0-system signals and the corresponding
1-system signals.
[0165] The intensity level adjustment value calculating unit 3062
is a calculating unit that detects an intensity level difference
between the 0-system and the 1-system delay adjustment signals, and
calculates a target value of the intensity level for adjusting the
intensity level difference to be constant.
[0166] The DCF characteristics data storage unit 3063 is a storage
unit such as a memory, and stores therein dispersion
characteristics data of the dispersion compensation fibers 3011 and
3014, such as the characteristic curve shown in FIG. 5. The
low-dispersion fiber buffer characteristics data storage unit 3064
is a storage unit such as a memory, and stores therein
characteristics data of the low-dispersion fiber buffers 303a and
303b. More particularly, the low-dispersion fiber buffer
characteristics data storage unit 3064 stores therein information
of a delay time, for example, generated by the optical fibers 3001a
through 3001e in each of the low-dispersion fiber buffers 303a and
303b in association with the corresponding optical fibers 3001a
through 3001e.
[0167] The setting information generating unit 3065 generates
information required for adjusting a delay of the optical signal to
the target value calculated by the delay adjustment value
calculating unit 3061. The information includes selection
information of the optical fibers 3001a through 3001e output to the
low-dispersion fiber buffers 303a and 303b, setting information of
the wavelength of the excitation light output to the one-step
wavelength converters 3010 and 3012 and the two-step wavelength
converters 3013 and 3015, dispersion compensation setting
information of the signal waveform output to the delay amount
constant dispersion compensating units 306a and 306b, intensity
level setting information of the optical single output to the
former-stage variable attenuators 308a and 308b, and delay
information of the optical signal output to the latter-stage
variable attenuators 310a and 310b.
[0168] More particularly, the setting information generating unit
3065 reads out characteristics data of the low-dispersion fiber
buffers 303a and 303b from the low-dispersion fiber buffer
characteristics data storage unit 3064. The setting information
generating unit 3065 generates selection information that is a
result of selection of the optical fibers 3001a through 3001e that
minimize a delay difference, when the delay difference exists
between the 0-system signal and the 1-system signal.
[0169] The setting information generating unit 3065, when there
still remains a delay difference between the 0-system signal and
the 1-system signal, and also in the 0-system signals or in the
1-system signals after the low-dispersion fiber buffers 303a and
303b adjust the delay difference, reads out information on
dispersion characteristics of the dispersion compensation fibers
3011 and 3014 from the DCF characteristics data storage unit 3063
to calculate a wavelength of the excitation light used by the
one-step wavelength converters 3010 and 3012 and the two-step
wavelength converters 3013 and 3015 that convert the
wavelengths.
[0170] In this case, the setting information generating unit 3065
calculates the wavelength of the excitation light so as to satisfy
the following formula (1).
T=Ta(.lamda.c1)+Tb(.lamda.c2),
S=(dTa(.lamda.)/d.lamda.|.sub..lamda.=.lamda.c1+dTb(.lamda.)/d.lamda.|.s-
ub..lamda.=.lamda.c2).DELTA.x (1)
[0171] In the above formula (1), T indicates a delay amount of the
entire signals of one of the 0-system and the 1-system, V indicates
a delay difference V in the signals of one of the 0-system and the
1-system, .lamda.c1 and .lamda.c2 respectively indicate the center
wavelengths of the wavelength bands of signals that pass through
the dispersion compensation fibers 3011 and 3014 shown in FIG. 3,
Ta and Tb indicate delay times generated when signals pass through
the dispersion compensation fibers 3011 and 3014, respectively, and
.DELTA.x indicates a width of a signal wavelength band.
[0172] When the low-dispersion fiber buffers 303a and 303b can
fully compensate the delay difference in the signals of the
0-system and the 1-system, respectively, it becomes V=0. The
setting information generating unit 3065 calculates the wavelength
of the excitation light used by the one-step wavelength converters
3010 and 3012 and the two-step wavelength converters 3013 and 3015,
so as to satisfy the following formula (2).
T=Ta(.lamda.c1)+Tb(.lamda.c2),
0=(dTa(.lamda.)/d.lamda.|.sub..lamda.=.lamda.c1+dTB(.lamda.)/d.lamda.|.s-
ub..lamda.=.lamda.c2).DELTA.x (2)
[0173] The setting information generating unit 3065 performs a
process of calculating a compensation amount D, when the delay
amount constant dispersion compensating units 306a and 306b
compensate a signal waveform distortion. More particularly, the
setting information generating unit 3065 calculates the
compensation amount D from the dispersion characteristics data of
the dispersion compensation fibers 3011 and 3014 stored in the DCF
characteristics data storage unit 3063, and information on the
wavelength of the excitation light.
[0174] The setting information generating unit 3065 further
performs a process of generating setting information on the
intensity level output to the former-stage variable attenuators
308a and 308b, based on the information on the target value of the
intensity level received from the intensity level adjustment value
calculating unit 3062. The setting information generating unit 3065
generates delay information that shows whether a delay difference
is absorbed between the 0-system signal and the 1-system signal, to
output to the latter-stage variable attenuators 310a and 310b. The
latter-stage variable attenuators 310a and 310b receive the delay
information, at an initial startup of the hitless protection
switching apparatus 30 and when the 0-system and the 1-system
recover from the signal interruption, to start a process of
adjusting an intensity level of an optical signal.
[0175] The interface units 3066, 3067, 3068, 3069, 3070, and 3071
are interfaces that output optical fiber selection information,
wavelength setting information, dispersion compensation setting
information, intensity level setting information, delay
information, and the like, to the low-dispersion fiber buffers 303a
and 303b, the one-step wavelength converters 3010 and 3012, the
two-step wavelength converters 3013 and 3015, the delay amount
constant dispersion compensating units 306a and 306b, the
former-stage variable attenuators 308a and 308b, and the
latter-stage variable attenuators 310a and 310b.
[0176] Next, a procedure of a startup process performed by the
hitless protection switching apparatus 30 will be explained. FIG.
19 is a flowchart showing a procedure of a startup process
performed by the hitless protection switching apparatus 30.
[0177] As shown in FIG. 19, the hitless protection switching
apparatus 30 performs an initial startup process, when the power is
turned ON (step S101). More particularly, the hitless protection
switching apparatus 30 starts the respective amplifiers 300a, 300b,
302a, 302b, 304a, 304b, 307a, and 307b. The low-dispersion fiber
buffers 303a and 303b select the optical fibers 3001a through 3001e
to be used temporarily, from the optical fibers 3001a through 3001
shown in FIG. 2.
[0178] The one-step wavelength converters 3010 and 3012, and the
two-step wavelength converters 3013 and 3015 of the
multi-wavelength light variable delay units 305a and 305b,
temporarily set a wavelength shift amount used when the wavelength
is converted. The delay amount constant dispersion compensating
units 306a and 306b temporarily set a compensation amount to
compensate a signal waveform distortion. The former-stage variable
attenuators 308a and 308b open a transmission route of a
signal.
[0179] The controller 314 confirms that the delay adjustment
signals of the 0-system and the 1-system are recovered from the
signal interruption state (step S102). Subsequently, the
low-dispersion fiber buffers 303a and 303b adjust the delay
difference, by selecting the optical fibers 3001a through 3001e,
based on the selection information on the optical fibers 3001a
through 3001e of the low-dispersion fiber buffers 303a and 303b,
transmitted from the controller 314 (step S103).
[0180] The controller 314 then checks whether the delay difference
of the delay adjustment signals of the 0-system or the 1-system can
be reduced, by using the low-dispersion fiber buffers 303a and 303b
(step S104). If the delay difference can be reduced (YES at step
S104), the low-dispersion fiber buffers 303a and 303b readjust the
delay difference (step S103).
[0181] If the delay difference cannot be reduced by the
low-dispersion fiber buffers 303a and 303b any more (NO at step
S104), the multi-wavelength light variable delay units 305a and
305b adjust the delay difference of the 0-system and the 1-system
(step S105). The delay amount constant dispersion compensating
units 306a and 306b adjust the dispersion value that corrects a
signal distortion, without changing the delay amount of the
respective signals of the 0-system and the 1-system (step
S106).
[0182] The former-stage variable attenuators 308a and 308b adjust
the intensity levels of the signals of the 0-system and the
1-system to be constant (step S107). The controller 314 checks
whether the delay difference of the delay adjustment signals of the
0-system and the 1-system is absorbed (step S108). If the delay
difference is not absorbed (NO at step S108), the multi-wavelength
light variable delay units 305a and 305b perform the process of
readjusting the delay of the 0-system and the 1-system (step
S105).
[0183] When the delay difference is absorbed (YES at step S108),
the latter-stage variable attenuators 310a and 310b adjust the
intensity levels of the signals of the 0-system and the 1-system to
be constant (step S109), and finish the startup process of the
hitless protection switching apparatus 30.
[0184] Next, a procedure of a signal intensity level adjustment
process performed when a signal interruption occurs, will be
explained. FIG. 20 is a flowchart showing a procedure of a signal
intensity level adjustment process performed when a signal
interruption occurs.
[0185] As shown in FIG. 20, the light interruption detecting unit
311 of the hitless protection switching apparatus 30 executes a
monitor process to monitor the signals of the 0-system and the
1-system input into the amplifiers 300a and 300b (step S201).
[0186] The light interruption detecting unit 311 checks whether a
signal interruption has occurred (step S202). If the signal
interruption has not occurred (NO at step S202), the signal
interruption monitor process is continued (step S201).
[0187] If the signal interruption has occurred (YES at step S202),
the latter-stage variable attenuators 310a and 310b receive the
information from the light interruption detecting unit 311, adjust
the intensity levels of the signals as explained in FIG. 16 (step
S203), and finishes the signal intensity level adjustment
process.
[0188] The signal monitored in the monitor process may either be a
delay adjustment signal or a main signal. However, when the delay
adjustment signal is monitored, the signal interruption of the
0-system optical fiber line 50 or the 1-system optical fiber line
60 cannot be detected properly, if the branching apparatus 20 that
multiplexes the delay adjustment signal to the main signal breaks
down and cannot multiplex. Therefore, the delay adjustment signal
and the main signal are preferably both monitored.
[0189] Next, a procedure of a recovery process performed when the
hitless protection switching apparatus 30 recovers from a signal
interruption will be explained. FIG. 21 is a flowchart showing a
procedure of a recovery process performed by the hitless protection
switching apparatus 30, when recovering from a signal interruption.
A recovery of the 0-system from a signal interruption will be
explained here.
[0190] As shown in FIG. 21, the controller 314 of the hitless
protection switching apparatus 30 detects a 0-system delay
adjustment signal to detect that the 0-system has recovered from
the signal interruption (step S301). When the delay adjustment
signal is detected for more than a predetermined time (recovery
protection time), the controller 314 determines that the 0-system
has recovered from the signal interruption.
[0191] The low-dispersion fiber buffer 303a adjusts the delay
difference between the 0-system and the 1-system by selecting the
optical fibers 3001a through 3001e, based on the selection
information of the optical fibers 3001a through 3001e of the
low-dispersion fiber buffer 303a transmitted by the controller 314
(step S302).
[0192] The controller 314 then checks whether the delay difference
between the 0-system and the 1-system can be reduced, by using the
low-dispersion fiber buffers 303a and 303b (step S303). If the
delay difference can be reduced (YES at step S303), the
low-dispersion fiber buffer 303a readjusts the delay difference
between the 0-system and the 1-system (step S302).
[0193] If the delay difference cannot be reduced by the
low-dispersion fiber buffer 303a any more (NO at step S303), the
multi-wavelength light variable delay unit 305a adjusts the delay
difference between the 0-system and the 1-system (step S304). The
delay amount constant dispersion compensating unit 306a then
adjusts the dispersion value that corrects the signal distortion of
the 0-system, without changing the delay amount of the respective
signals of the 0-system and the 1-system (step S305).
[0194] The former-stage variable attenuator 308a adjusts the
intensity level of the 0-system signal so as to be the same level
as the intensity level of the 1-system signal (step S306). The
controller 314 checks whether the delay difference of the delay
adjustment signals of the 0-system and the 1-system is absorbed
(step S307). If the delay difference is not absorbed (NO at step
S307), the multi-wavelength light variable delay unit 305a performs
the process of readjusting the delay of the 0-system and the
1-system (step S304).
[0195] If the delay is absorbed (YES at step S307), the
latter-stage variable attenuators 310a and 310b adjust the
intensity levels of the signals of the 0-system and the 1-system to
be constant (step S308), and finishes the recovery process of this
hitless protection switching apparatus 30.
[0196] As described above, according to the first embodiment, the
multi-wavelength light variable delay units 305a and 305b convert
the wavelengths of optical signals, and adjust the transmission
delay difference between the optical signals of the 0-system and
the 1-system, by making the optical signals with a converted
wavelength pass through the dispersion compensation fibers 3011 and
3014, in which the transmission delay of the optical signal changes
continuously depending on the wavelength. Further, the delay amount
constant dispersion compensating units 306a and 306b compensate
waveform degradation of the optical signals, while maintaining the
adjusted transmission delay difference. As a result, the
transmission delay difference of the optical signals transmitted
via the redundant routes of the 0-system and the 1-system can be
adjusted continuously, while preventing degradation of the optical
signal.
[0197] According to the first embodiment, the multi-wavelength
light variable delay units 305a and 305b, when a wavelength of an
optical signal is converted, reconvert the wavelength of the
optical signal into a predetermined wavelength. As a result, the
wavelength of the optical signal with a converted wavelength can be
returned to the original wavelength.
[0198] According to the first embodiment, the low-dispersion fiber
buffers 303a and 303b adjust the transmission delay difference of
the optical signal by switching the optical fibers 3001a through
3001e with different lengths, and discretely changing the
transmission delay of the optical signal. The multi-wavelength
light variable delay units 305a and 305b then convert the
wavelength of the optical signal, and adjust the transmission delay
difference between the optical signals of the 0-system and the
1-system, by making the optical signal with a converted wavelength
pass through the dispersion compensation fibers 3011 and 3014 in
which the transmission delay of the optical signal changes
continuously depending on the wavelength. As a result, the
transmission delay difference of the optical signal can be broadly
adjusted, by discretely changing the transmission delay, and then
the transmission delay difference of the optical signal can be
refined, by continuously changing the transmission delay
afterwards.
[0199] According to the first embodiment, the multi-wavelength
light variable delay units 305a and 305b adjust the transmission
delay difference of plural optical signals, by signals with the
same wavelengths, when the plural optical signals are transmitted
via the redundant routes of the 0-system and the 1-system and
multiplexed by the wavelength division multiplexing. As a result,
the transmission delay difference between the multiplexed signals
with the same wavelength can be adjusted, using the wavelength
division multiplexing.
[0200] According to the first embodiment, the wavelength separating
units 309a and 309b receive the optical signal of which the delay
difference detecting signal used for detecting the transmission
delay difference of the optical signal is being multiplexed. The
controller 314 and the multi-wavelength light variable delay units
305a and 305b adjust the transmission delay difference between the
optical signals, by referring to the delay difference detecting
signal. As a result, the transmission delay difference between the
optical signals can efficiently be adjusted, by multiplexing the
delay difference detecting signal to the optical signal.
[0201] According to the first embodiment, the multiplexer 313
combines and outputs the optical signals at the 0-system and the
1-system routes, and when the light interruption detecting unit 311
detects a signal interruption, the latter-stage variable
attenuators 310a and 310b adjust the intensity levels of the
optical signals at the 0-system and the 1-system routes, in which
the waveform degradation is compensated so that the intensity level
of the optical signal output from the multiplexer 313 is of a
predetermined level. As a result, when a signal interruption
occurs, a fluctuation of the intensity level of the optical signal
can be effectively suppressed.
[0202] According to the first embodiment, the low-dispersion fiber
buffers 303a and 303b adjust the transmission delay difference of
the optical signal, by switching the optical fibers 3001a through
3001e with different lengths, and discretely changing the
transmission delay of the optical signal. The multi-wavelength
light variable delay units 305a and 305b convert the wavelength of
the optical signal, and adjust the delay difference of the optical
signal, by making the optical signal with a converted wavelength
pass through the dispersion compensation fibers 3011 and 3014 that
generate the transmission delay depending on the wavelength of the
optical signal. The light interruption detecting unit 311 detects a
signal interruption generated to the signal before the optical
signal is input into the low-dispersion fiber buffers 303a and
303b. As a result, a fluctuation of the intensity level of the
optical signal can be effectively suppressed, by detecting the
signal interruption early, before the transmission delay difference
is adjusted.
[0203] According to the first embodiment, when a signal
interruption is detected by the light interruption detecting unit
311, the latter-stage variable attenuators 310a and 310b adjust the
intensity levels of the optical signals of the respective routes,
before the optical signals stop reaching the latter-stage variable
attenuators 310a and 310b. As a result, the transmission of the
optical signals can be prevented from a momentary interruption.
[0204] The embodiment of the present invention has been explained,
but the present invention may be applied to various different
embodiments, other than the first embodiment. In the following,
other embodiments included in the present invention will be
explained as a second embodiment.
(1) Transmission Route of Optical Signal Whose Delay Amount is
Adjusted
[0205] For example, according to the first embodiment, as shown in
FIG. 1, the delay amount is adjusted with respect to the both
signals of the 0-system and the 1-system. Alternatively, the delay
difference between the 0-system signal and the 1-system signal may
be absorbed, by adjusting the delay amount only for the signals of
one of the 0-system and the 1-system.
[0206] FIG. 22 is a functional configuration diagram of a hitless
protection switching system that performs an adjustment of a delay
amount only for a 0-system signal. The functioning units as those
of the functioning units shown in FIG. 1 are denoted by the same
reference numerals, and the detailed description thereof will not
be repeated.
[0207] As shown in FIG. 22, the hitless protection switching system
includes the WDM transmission apparatus 10, the branching apparatus
20, a hitless protection switching apparatus 70, and the WDM
transmission apparatus 40. The branching apparatus 20 and the
hitless protection switching apparatus 70 are connected via the
0-system optical fiber line 50 and the 1-system optical fiber line
60.
[0208] The WDM transmission apparatus 10, the branching apparatus
20, and the WDM transmission apparatus 40 are the same apparatuses
as the WDM transmission apparatus 10, the branching apparatus 20,
and the WDM transmission apparatus 40 that are explained in FIG. 1.
The hitless protection switching apparatus 70, compared with the
hitless protection switching apparatus 30 shown in FIG. 1, does not
include the amplifier 304b, the multi-wavelength light variable
delay unit 305b, and the delay amount constant dispersion
compensating unit 306b that are in the 1-system, and is configured
to perform an adjustment of the delay amount only for the 0-system
signal.
[0209] In this manner, the hitless protection switching apparatus
70 adjusts the delay amount of the 0-system signal, and absorbs the
delay difference between the 1-system signal. Accordingly,
similarly to the hitless protection switching apparatus 30 in FIG.
1, the transmission of the optical signals can be continued without
interruption, even if a signal interruption occurs.
(2) Number of Applied Dispersion Compensation Fibers
[0210] According to the first embodiment, as shown in FIG. 3, the
two dispersion compensation fibers 3011 and 3014 are used to absorb
the delay difference between the 0-system signal and the 1-system
signal. However, if the delay difference between the respective
0-system multiplexed signals and the delay difference between the
respective 1-system multiplexed signals are adjusted to be equal in
the low-dispersion fiber buffers 303a and 303b, only the delay
difference between the 0-system signal and the 1-system signal may
be adjusted. Accordingly, only one dispersion compensation fiber is
enough.
[0211] FIG. 23 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes only one
dispersion compensation fiber. As shown in FIG. 23, this
multi-wavelength light variable delay unit includes two-step
wavelength converters 3080 and 3082, a dispersion compensation
fiber 3081, and a temperature controller 3083.
[0212] The two-step wavelength converters 3080 and 3082, the
dispersion compensation fiber 3081, and the temperature controller
3083 are the same as the two-step wavelength converters 3013 and
3015, the dispersion compensation fiber 3014, and the temperature
controller 3017 of the multi-wavelength light variable delay units
305a and 305b shown in FIG. 3. The difference between the
multi-wavelength light variable delay unit shown in FIG. 23 and the
multi-wavelength light variable delay units 305a and 305b shown in
FIG. 3, is that the one-step wavelength converters 3010 and 3012,
and the dispersion compensation fiber 3011 are removed.
[0213] In this manner, the delay difference between the respective
0-system multiplexed signals and the delay difference between the
respective 1-system multiplexed signals in the low-dispersion fiber
buffers 303a and 303b may be adjusted so as to be the same.
Accordingly, a configuration of the multi-wavelength light variable
delay unit can be simplified.
[0214] The two-step wavelength converters 3080 and 3082, and the
dispersion compensation fiber 3081 are used here. Alternatively,
however, a multi-wavelength light variable delay unit can be formed
by using the one-step wavelength converters 3010 and 3012, and the
dispersion compensation fiber 3081.
(3) Setting Position of Two-Step Wavelength Converter 3015
[0215] According to the first embodiment, as shown in FIG. 3, the
two-step wavelength converter 3015 of the multi-wavelength light
variable delay units 305a and 305b adjust the wavelength of each
signal so as to be the same as the wavelength at the time of input
into the multi-wavelength light variable delay units 305a and 305b.
Alternatively, however, the adjustment may be made after the
0-system signal and the 1-system signal are combined by the
multiplexer 313.
[0216] FIG. 24 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes only one
two-step wavelength converter 3013. As shown in FIG. 24, this
multi-wavelength light variable delay unit includes the one-step
wavelength converters 3010 and 3012, the dispersion compensation
fibers 3011 and 3014, the two-step wavelength converter 3013, and
the temperature controllers 3016 and 3017.
[0217] The one-step wavelength converters 3010 and 3012, the
dispersion compensation fibers 3011 and 3014, the two-step
wavelength converter 3013, and the temperature controllers 3016 and
3017, are the same as the one-step wavelength converters 3010 and
3012, the dispersion compensation fibers 3011 and 3014, the
two-step wavelength converter 3013, and the temperature controllers
3016 and 3017 shown in FIG. 3. The difference between the
multi-wavelength light variable delay unit shown in FIG. 24 and the
multi-wavelength light variable delay units 305a and 305b shown in
FIG. 3, is that the two-step wavelength converter 3015 is
removed.
[0218] FIG. 25 is an explanatory diagram of a setting location of
the two-step wavelength converter 3015. As shown in FIG. 25, the
two-step wavelength converter 3015 removed from the
multi-wavelength light variable delay unit is set behind the
multiplexer 313. The latter-stage variable attenuators 310a and
310b and the multiplexer 313 shown in FIG. 25 are the same as the
latter-stage variable attenuators 310a and 310b and the multiplexer
313 shown in FIG. 1. With this, similarly to that shown in FIG. 3,
the wavelength of each signal can be adjusted so as to be the same
as the wavelength at the time of input into the multi-wavelength
light variable delay units 305a and 305b.
[0219] This is the same when the multi-wavelength light variable
delay unit includes only one dispersion compensation fiber, as
shown in FIG. 23. FIG. 26 is a functional configuration diagram of
a multi-wavelength light variable delay unit that includes one each
of the two-step wavelength converter 3080 and the dispersion
compensation fiber 3081.
[0220] As shown in FIG. 26, this multi-wavelength light variable
delay unit includes one each of the two-step wavelength converter
3080 and the dispersion compensation fiber 3081. The two-step
wavelength converter that adjusts the wavelength of each signal so
as to be the same as the wavelength at the time of input into the
two-step wavelength converter 3080 is set after the multiplexer
313, as explained in FIG. 25.
(4) Characteristics of Dispersion Compensation Fiber
[0221] According to the first embodiment, as shown in FIG. 3, the
two dispersion compensation fibers 3011 and 3014 of the
multi-wavelength light variable delay unit have the same dispersion
slope as shown in FIG. 5. However, the two dispersion compensation
fibers 3011 and 3014 may have reversed dispersion slopes.
[0222] FIG. 27 is a functional configuration diagram of a
multi-wavelength light variable delay unit that includes two-step
wavelength converters 3090 and 3092, and dispersion compensation
fibers 3091 and 3093 that have reversed dispersion slopes to each
other. The two-step wavelength converters 3090 and 3092 are the
same as the two-step wavelength converters 3013 and 3015 shown in
FIG. 3. Another two-step wavelength converter may further be
provided for adjusting the wavelength of each signal to be the same
as the wavelength at the time of input into the two-step wavelength
converter 3090.
[0223] The dispersion compensation fibers 3091 and 3093 are optical
fibers that have such dispersion characteristics that the
transmission speed of an optical signal that passes through the
dispersion compensation fibers 3091 and 3093 changes depending on a
wavelength of the optical signal, and have reversed dispersion
slopes to each other.
[0224] In other words, the delay time of the dispersion
compensation fiber 3091 increases along the increase of the
wavelength, as shown in FIG. 5. On the other hand, the other
dispersion compensation fiber 3093 has such a characteristic that
the delay time decreases along the increase of the wavelength.
[0225] When signals with different wavelengths that have a delay
difference are input to the two-step wavelength converter 3090, the
wavelengths are converted by the two-step wavelength converter
3090. In this case, as explained in FIG. 7, the magnitude
relationship of the wavelength between the signals does not change.
When these signals pass through the dispersion compensation fiber
3091, the signals with a larger wavelength delay more.
[0226] The two-step wavelength converter 3092 then converts the
wavelengths of the signals that have passed through the dispersion
compensation fiber 3091. Also in this case, the magnitude
relationship of the wavelengths among the signals does not change.
However, if the signals whose wavelengths are converted by the
two-step wavelength converter 3092 are made to pass through the
dispersion compensation fiber 3093, the signals with a smaller
wavelength delay more. This is because the dispersion compensation
fiber 3093 has the reversed dispersion slope to the dispersion
compensation fiber 3091.
[0227] In this manner, the delay amount of the signal can be
adjusted, by combining and using the dispersion compensation fibers
3091 and 3093 having the reversed dispersion slopes.
[0228] In FIG. 27, the wavelength is converted by using the
two-step wavelength converters 3090 and 3092. Alternatively,
however, the wavelength may be converted by using a one-step
wavelength converter and a two-step wavelength converter. FIG. 28
is a functional configuration diagram of a multi-wavelength light
variable delay unit that includes a one-step wavelength converter
3094, a two-step wavelength converter 3096, and the dispersion
compensation fibers 3095 and 3097 that have reversed dispersion
slopes to each other.
[0229] The one-step wavelength converter 3094 and the two-step
wavelength converter 3096 are the same as the one-step wavelength
converters 3010 and 3012, and the two-step wavelength converters
3013 and 3015 shown in FIG. 3. Another two-step wavelength
converter may further be provided for adjusting the wavelength of
each signal to be the same as the wavelength at the time of input
into the two-step wavelength converter 3090. Dispersion
compensation fibers 3095 and 3097 are the same as the dispersion
compensation fibers 3091 and 3093 shown in FIG. 27, and have
reversed dispersion slopes to each other.
[0230] When the signals with different wavelengths that have a
delay difference are input into the one-step wavelength converter
3094, the wavelengths are converted by the one-step wavelength
converter 3094. In this case, the magnitude relationship of the
wavelengths between the signals reverses. When these signals pass
through the dispersion compensation fiber 3095, the signals whose
wavelengths were smaller when input into the one-step wavelength
converter 3094, delay more.
[0231] The two-step wavelength converter 3096 then converts the
wavelengths of the signals that have passed through the dispersion
compensation fiber 3095. In this case, the magnitude relationship
of the wavelengths between the signals does not change. However, if
the signals whose wavelengths are converted by the two-step
wavelength converter 3096 are made to pass through the dispersion
compensation fiber 3097, the signals whose wavelengths were larger
when input into the one-step wavelength converter 3094, delay more.
This is because the dispersion compensation fiber 3097 has the
reversed dispersion slope to the dispersion compensation fiber
3095.
[0232] In this manner, even in FIG. 28, the delay amount of the
signal can be adjusted, by combining and using the dispersion
compensation fibers 3095 and 3097 that have the reversed dispersion
slopes.
(5) Manner of Detection of Signal Interruption
[0233] According to the first embodiment, as show in FIG. 1, the
controller 314 adjusts the delay amount by detecting the delay
adjustment signal. However, the delay amount may be adjusted by
detecting a communication error signal, when an optical signal is
converted into an electric signal, after the 0-system optical
signal and the 1-system optical signal are combined. The conversion
is made based on communications standards such as a synchronous
optical network/synchronous digital hierarchy (SONET/SDH).
[0234] More particularly, the communication error signal is
generated when a phase difference exists between a 0-system signal
and a 1-system electric signal. The adjustment of the delay amount
is performed until the communication error signal is not generated
any more.
[0235] FIG. 29 is a functional configuration diagram of a hitless
protection switching system that adjusts a delay amount between the
0-system signal and the 1-system signal by monitoring a
communication error. This hitless protection switching system
includes an optical transmitter 80, an optical coupler 90, a
hitless protection switching apparatus 100, and an optical receiver
110. The optical coupler 90 and the hitless protection switching
apparatus 100 are connected via a 0-system optical fiber line 120
and a 1-system optical fiber line 130.
[0236] The optical transmitter 80 is a transmitter that transmits
an optical signal. The optical coupler 90 is an optical coupler
that branches the signal received from the optical transmitter 80,
and transmits the resulting signal via the 0-system optical fiber
line 120 and the 1-system optical fiber line 130. The optical
receiver 110 is a receiver that receives the optical signal
transmitted from the optical transmitter 80 via the hitless
protection switching apparatus 100.
[0237] The hitless protection switching apparatus 100 is an
apparatus that continues to transmit optical signals without
interruption, even if a failure occurs to one of the 0-system
optical fiber line 120 and the 1-system optical fiber line 130, by
using the optical signal of the other optical fiber line.
[0238] The hitless protection switching apparatus 100 converts the
wavelength of the optical signals that have passed through the
0-system optical fiber line 120 and the 1-system optical fiber line
130. By making the optical signal pass through an optical fiber
that has wavelength-dependent transmission speed of an optical
signal, the transmission delay difference of the optical signal can
be adjusted continuously. The hitless protection switching
apparatus 100, after adjusting the transmission delay difference of
the optical signal, also compensates waveform distortion of the
optical signal while maintaining the transmission delay
difference.
[0239] The hitless protection switching apparatus 100 includes
amplifiers 1000a and 1000b, dispersion compensating units 1001a and
1001b, amplifiers 1002a and 1002b, light variable delay units 1003a
and 1003b, former-stage variable attenuators 1004a and 1004b,
optical couplers 1005a and 1005b, latter-stage variable attenuators
1006a and 1006b, a light interruption detecting unit 1007, a level
adjusting unit 1008, an optical coupler 1009, an optical coupler
1010, an optical/electrical converter 1011, an error monitoring
unit 1012, and a delay adjusting unit 1013.
[0240] The amplifiers 1000a, 1000b, 1002a, and 1002b are amplifiers
that amplify an optical signal. The dispersion compensating units
1001a and 1001b are processing units that compensate wavelength
dispersion of the optical signal that has passed through the
0-system optical fiber line 120 and the 1-system optical fiber line
130, and formed by the DCF and the like.
[0241] The light variable delay units 1003a and 1003b are
processing units that convert the wavelength of an optical signal,
and continuously adjust the transmission delay of the optical
signal, by making the optical signal pass through an optical fiber
that has wavelength-dependent transmission speed of an optical
signal. The light variable delay units 1003a and 1003b, after
adjusting the transmission delay of the optical signal, amplify and
output the optical signal by compensating the waveform distortion
of the optical signal while maintaining the transmission delay.
[0242] The light variable delay units 1003a and 1003b are
functioning units that correspond to the low-dispersion fiber
buffers 303a and 303b, the amplifiers 304a and 304b, the
multi-wavelength light variable delay units 305a and 305b, the
delay amount constant dispersion compensating units 306a and 306b,
and the amplifiers 307a and 307b in the hitless protection
switching apparatus 30 shown in FIG. 1.
[0243] The former-stage variable attenuators 1004a and 1004b are
processing units that adjust an intensity level of a signal. More
particularly, the former-stage variable attenuators 1004a and 1004b
adjust the intensity levels of the signals of the 0-system and the
1-system to be the same, under the control of a controller (not
shown).
[0244] The optical couplers 1005a and 1005b are optical couplers
that branch an optical signal. The optical couplers 1005a and 1005b
branch the optical signal received from the former-stage variable
attenuators 1004a and 1004b, and output to the latter-stage
variable attenuators 1006a and 1006b, and the optical coupler
1010.
[0245] The latter-stage variable attenuators 1006a and 1006b are
processing units that adjust an intensity level of a signal,
similarly to the former-stage variable attenuators 1004a and 1004b.
The latter-stage variable attenuators 1006a and 1006b adjust the
signal output from the hitless protection switching apparatus 100,
so as to eliminate a fluctuation of the intensity level, as
explained in FIG. 16, when a signal interruption occurs in one of
the 0-system optical fiber line 120 and in the 1-system optical
fiber line 130.
[0246] The light interruption detecting unit 1007 is a processing
unit that detects whether a signal interruption has occurred in one
of the 0-system optical fiber line 120 and the 1-system optical
fiber line 130, by monitoring the signal input to the amplifiers
1000a and 1000b. If a signal interruption is detected, the light
interruption detecting unit 1007 notifies the level adjusting unit
1008 of the signal interruption.
[0247] The level adjusting unit 1008 is an adjusting unit that
controls the latter-stage variable attenuators 1006a and 1006b, and
adjusts an output level of the signal at the latter-stage variable
attenuators 1006a and 1006b, as explained in FIG. 16, when the
signal interruption is notified from the light interruption
detecting unit 1007. The optical coupler 1009 is an optical coupler
that combines the 0-system signal and the 1-system signal.
[0248] The optical coupler 1010 is an optical coupler that combines
the optical signals received from the optical couplers 1005a and
1005b. The optical/electrical converter 1011 is a processing unit
that converts the optical signal received from the optical coupler
1010 to an electric signal, based on the communications standards
such as the SONET/SDH. The optical/electrical converter 1011
generates an electric signal that includes a communication error
signal, when the delay difference exists between the 0-system
signal and the 1-system signal that are combined by the optical
coupler 1010.
[0249] The error monitoring unit 1012 is a processing unit that
monitors whether a communication error signal is included in the
electric signal generated by the optical/electrical converter 1011,
and notifies the delay adjusting unit 1013 of the monitoring
result.
[0250] The delay adjusting unit 1013 is a processing unit that
receives information on the monitoring result indicating whether
the communication error signal is included in the electric signal
from the error monitoring unit 1012, and absorbs the delay
difference between the 0-system signal and the 1-system signal.
This is enabled by adjusting the delay amount of the 0-system
signal and the 1-system signal until the communication error signal
is not detected any more, by controlling the light variable delay
units 1003a and 1003b.
[0251] In this manner, when the optical/electrical converter 1011
converts the optical signal into the electric signal, based on the
communications standards such as the SONET/SDH, the error
monitoring unit 1012 detects the communication error signal
generated when the delay difference exists in the optical signals
of a plurality of routes. The light variable delay units 1003a and
1003b also adjust the transmission delay difference between the
optical signals of the respective routes until the communication
error signal is not detected any more. Accordingly, the adjustment
of the transmission delay difference of the optical signal can be
performed with ease, by detecting the communication error signal
instead of detecting the delay difference itself.
[0252] In the configuration of the hitless protection switching
apparatus 100 in FIG. 29, the dispersion compensating unit 1001a
and 1001b and the amplifiers 1002a and 1002b may be removed so that
the signals output from the amplifiers 1000a and 1000b are directly
input into the light variable delay units 1003a and 1003b.
(6) Simple Configuration of Hitless Protection Switching
Apparatus
[0253] According to the first embodiment, the configuration of the
hitless protection switching apparatus is as shown in FIG. 1.
However, depending on an extent of the delay difference between the
0-system signal and the 1-system signal, and an extent of the
signal waveform distortion, the dispersion compensating units 301a
and 301b, the amplifiers 302a and 302b, the low-dispersion fiber
buffers 303a and 303b, and the amplifiers 304a and 304b shown in
FIG. 1 may appropriately be removed.
(7) Route Switching Process Performed by Using Optical Switch
[0254] According to the first embodiment, the transmission of the
optical signals is continued without interruption even if a signal
interruption occurs, by controlling the latter-stage variable
attenuators 310a and 310b shown in FIG. 1. However, the
transmission route may be switched using an optical switch, if some
signal interruption cannot be a matter.
[0255] FIG. 30 is a functional configuration diagram of a route
switching system that performs a route switching process using an
optical switch. As shown in FIG. 30, this route switching system
includes optical transmitters 400a through 400d, a WDM transmission
apparatus 410, a branching apparatus 420, a route switching
apparatus 430, a WDM transmission apparatus 440, and optical
receivers 450a through 450d. The branching apparatus 420 and the
route switching apparatus 430 are connected via a 0-system optical
fiber line 460 and a 1-system optical fiber line 470.
[0256] The optical transmitters 400a through 400d are transmitters
that transmit an optical signal. The WDM transmission apparatus 410
is an apparatus that multiplexes the optical signals transmitted
from the optical transmitters 400a through 400d, using the
wavelength division multiplexing, and transmits the multiplexed
optical signals with different wavelengths.
[0257] The branching apparatus 420 is an apparatus that multiplexes
an optical signal for delay adjustment to the optical signal
received from the WDM transmission apparatus 410, and then branches
and transmits the resulting optical signal. The branching apparatus
420 includes a delay adjustment light source 4200, a wavelength
multiplexing unit 4201, and a branching unit 4202. The delay
adjustment light source 4200, the wavelength multiplexing unit
4201, and the branching unit 4202, are the same as the delay
adjustment light source 200, the wavelength multiplexing unit 201,
and the branching unit 202 explained in FIG. 1.
[0258] The WDM transmission apparatus 440 is an apparatus that
receives the multiplexed optical signals with different wavelengths
from the route switching apparatus 430, separates the optical
signals by each wavelength, and transmits the resulting signals to
the optical receivers 450a through 450d. The optical receivers 450a
through 450d are receivers that receive the optical signals
transmitted from the optical transmitters 400a through 400d, via
the route switching apparatus 430.
[0259] The route switching apparatus 430 is an apparatus that, when
a signal interruption occurs to one of the 0-system optical fiber
line 460 and the 1-system optical fiber line 470, switches the
communication route of the signal to the other of the 0-system
optical fiber line 460 and to the 1-system optical fiber line 470
without signal interruption.
[0260] The route switching apparatus 430 also converts the
wavelength of the optical signal that has passed through the
0-system optical fiber line 460 and the 1-system optical fiber line
470. By making the optical signal pass through an optical fiber
that has a wavelength-dependent transmission speed of an optical
signal, the transmission delay difference of the optical signal can
be adjusted continuously. The route switching apparatus 430, after
adjusting the transmission delay difference of the optical signal,
compensates the waveform distortion of the optical signal while
maintaining the transmission delay difference.
[0261] The route switching apparatus 430 includes amplifiers 4300a
and 4300b, dispersion compensating units 4301a and 4301b,
amplifiers 4302a and 4302b, light variable delay units 4303a and
4303b, wavelength separating units 4304a and 4304b, an optical
switch controller 4305, an optical switch 4306, and a controller
4307.
[0262] The amplifiers 4300a, 4300b, 4302a, and 4302b are amplifiers
that amplify an optical signal. The dispersion compensating units
4301a and 4301b are processing units that compensate the wavelength
dispersion of the optical signal that has passed through the
0-system optical fiber line 460 and the 1-system optical fiber line
470, and formed by the DCF and the like.
[0263] The light variable delay units 4303a and 4303b are
processing units that convert the wavelength of an optical signal,
and continuously adjust the transmission delay difference of the
optical signal, by making the optical signal pass through an
optical fiber that has wavelength-dependent transmission speed of
an optical signal. The light variable delay units 4303a and 4303b,
after adjusting the transmission delay difference of the optical
signal, compensate the waveform distortion of the optical signal
while maintaining the transmission delay difference, and amplify
and output the optical signal.
[0264] The light variable delay units 4303a and 4303b are
functioning units that correspond to the low-dispersion fiber
buffers 303a and 303b, the amplifiers 304a and 304b, the
multi-wavelength light variable delay units 305a and 305b, the
delay amount constant dispersion compensating units 306a and 306b,
and the amplifiers 307a and 307b in the hitless protection
switching apparatus 30 shown in FIG. 1.
[0265] The wavelength separating units 4304a and 4304b are
separating units that separate two delay adjustment signals with
different wavelengths included in each of the 0-system signal and
the 1-system signal, and transmit to the controller 4307.
[0266] The optical switch controller 4305 is a processing unit that
detects whether a signal interruption has occurred in one of the
0-system optical fiber line 460 and the 1-system optical fiber line
470, by monitoring the signals output from the amplifiers 4300a and
4300b. If the signal interruption is detected, the optical switch
controller 4305 switches the transmission route of the signal to
one of the 0-system and the 1-system without signal interruption,
by controlling the optical switch 4306. The optical switch 4306 is
an optical switch that switches the transmission route of the
signal output to the WDM transmission apparatus 440, between the
0-system and the 1-system.
[0267] The controller 4307 is a controller that controls the light
variable delay units 4303a and 4303b, and adjusts a delay
difference between the 0-system signal and the 1-system signal. The
controller 4307 corresponds to the controller 314 shown in FIG.
1.
[0268] In this manner, when the route is switched between the
0-system and the 1-system, the route may be switched using the
optical switch 4306, when some signal interruption cannot be a
matter.
[0269] In the configuration of the route switching apparatus 430 in
FIG. 30, the route switching apparatus 430 may be formed so that
the signals output from the amplifiers 4300a and 4300b are directly
input into the light variable delay units 4303a and 4303b, by
removing the dispersion compensating units 4301a and 4301b, and
amplifiers 4302a and 4302b.
[0270] The optical switch controller 4305 may also detect whether a
signal interruption has occurred in one of the 0-system optical
fiber line 460 and the 1-system optical fiber line 470, by
monitoring the signals output from the amplifiers 4302a and
4302b.
(8) Separation Output Process of Multiplexed Optical Signal
[0271] According to the first embodiment, the multiplexer 313 of
the hitless protection switching apparatus 30 outputs the signals
that have passed through one of the 0-system and the 1-system in a
multiplexed state. Alternatively, however, the hitless protection
switching apparatus may be formed so as to separate and output the
respective signals.
[0272] FIG. 31 is a functional configuration diagram of a hitless
protection switching system that separates and outputs respective
signals. This hitless protection switching system includes optical
transmitters 500a through 500d, a WDM transmission apparatus 510, a
branching apparatus 520, a hitless protection switching apparatus
530, and an optical receiver 540. The branching apparatus 520 and
the hitless protection switching apparatus 530 are connected via a
0-system optical fiber line 550 and a 1-system optical fiber line
560.
[0273] The optical transmitters 500a through 500d are transmitters
that transmit an optical signal. The WDM transmission apparatus 510
is an apparatus that multiplexes the optical signals transmitted
from the optical transmitters 500a through 500d using the
wavelength division multiplexing, and transmits the multiplexed
optical signals with different wavelengths.
[0274] The branching apparatus 520 is an apparatus that multiplexes
an optical signal for delay adjustment to the optical signal
received from the WDM transmission apparatus 510, and branches and
transmits the resulting optical signal. The branching apparatus 520
includes a delay adjustment light source 5200, a wavelength
multiplexing unit 5201, and a branching unit 5202. The delay
adjustment light source 5200, the wavelength multiplexing unit
5201, and the branching unit 5202 are the same as the delay
adjustment light source 200, the wavelength multiplexing unit 201,
and the branching unit 202 explained in FIG. 1.
[0275] The optical receiver 540 is a receiver that receives optical
signals transmitted by the optical transmitters 500a through 500d
via the hitless protection switching apparatus 530. In FIG. 31,
only one optical receiver 540 is shown. However, the number of the
optical receiver 540 that receives the signal output from the
hitless protection switching apparatus 530 is arbitrary.
[0276] The hitless protection switching apparatus 530 is an
apparatus that continues the transmission of optical signals
without interruption, even if a failure occurs to one of the
0-system optical fiber line 550 and the 1-system optical fiber line
560, by using the optical signal of the other optical fiber line.
This hitless protection switching apparatus 530 also performs a
process of separating and outputting the multiplexed signals.
[0277] The hitless protection switching apparatus 530 converts the
wavelengths of the optical signals that have passed through the
0-system optical fiber line 550 and the 1-system optical fiber line
560. By making the optical signal pass through an optical fiber
that has wavelength-dependent transmission speed of an optical
signal, the transmission delay difference of the optical signal can
be adjusted continuously. The hitless protection switching
apparatus 530, after adjusting the transmission delay difference of
the optical system, compensates the waveform distortion of the
optical signal while maintaining the transmission delay
difference.
[0278] This hitless protection switching apparatus 530 includes
amplifiers 5300a and 5300b, dispersion compensating units 5301a and
5301b, amplifiers 5302a and 5302b, light variable delay units 5303a
and 5303b, wavelength separating units 5304a and 5304b, wavelength
separating units 5305a and 5305b, variable attenuators 5306a and
5306b, a light interruption detecting unit 5307, a level adjusting
unit 5308, a multiplexer 5309, and a controller 5310.
[0279] The amplifiers 5300a, 5300b, 5302a, and 5302b are amplifiers
that amplify an optical signal. The dispersion compensating units
5301a and 5301b are processing units that compensate the wavelength
dispersion of the optical signal that has passed through the
0-system optical fiber line 550 and the 1-system optical fiber line
560, and formed by the DCF, for example.
[0280] The light variable delay units 5303a and 5303b are
processing units that convert the wavelength of an optical signal,
and continuously adjust the transmission delay difference of the
optical signal, by making the optical signal pass through an
optical fiber that has wavelength-dependent transmission speed of
an optical signal. The light variable delay units 5303a and 5303b,
after adjusting the transmission delay difference of the optical
signal, compensate the waveform distortion of the optical signal
while maintaining the transmission delay difference, and amplify
and output the optical signal.
[0281] The light variable delay units 5303a and 5303b are
functioning units that correspond to the low-dispersion fiber
buffers 303a and 303b, the amplifiers 304a and 304b, the
multi-wavelength light variable delay units 305a and 305b, the
delay amount constant dispersion compensating units 306a and 306b,
and the amplifiers 307a and 307b in the hitless protection
switching apparatus 30 shown in FIG. 1.
[0282] The wavelength separating units 5304a and 5304b are
separating units that separate two delay adjustment signals with
different wavelengths included in each of the 0-system signal and
the 1-system signal, and transmit to the controller 5310.
[0283] The wavelength separating units 5305a and 5305b separate the
respective signals (Ch1, Ch2, Ch3, and Ch4) with different
wavelengths that respectively pass through the 0-system and the
1-system. The wavelength separating units 5305a and 5305b output
the respective separated signals to the variable attenuators 5306a
and 5306b. In FIG. 31, only two variable attenuators 5306a and
5306b are shown. However, the variable attenuators 5306a and 5306b
exist by the same number as the number of signals that are
separated by the wavelength separating units 5305a and 5305b.
[0284] The variable attenuators 5306a and 5306b are processing
units that adjust an intensity level of a signal, as explained in
FIG. 16. The light interruption detecting unit 5307 is a processing
unit that detects whether a signal interruption has occurred in one
of the 0-system optical fiber line 550 and the 1-system optical
fiber line 560, by monitoring the signal input to the amplifiers
5300a and 5300b. If the signal interruption is detected, the light
interruption detecting unit 5307 notifies the level adjusting unit
5308 of the signal interruption.
[0285] The level adjusting unit 5308 is an adjusting unit that,
when the notification of the signal interruption is received from
the light interruption detecting unit 5307, controls the variable
attenuators 5306a and 5306b, and adjusts an output level of the
signal at the variable attenuators 5306a and 5306b, as explained in
FIG. 16.
[0286] The multiplexer 5309 is an optical coupler that combines the
0-system signal (Ch1) and the 1-system signal (Ch1) that are
separated by the wavelength separating units 5305a and 5305b. In
FIG. 31, only one multiplexer 5309 is shown. However, the
multiplexer 5309 exists by the same number as the number of the
signals (Ch1, Ch2, Ch3, and Ch4) to be combined.
[0287] By forming the hitless protection switching apparatus 530 as
such, the transmission of the optical signals can be continued
without interruption, even if a failure occurs in one of the
0-system optical fiber line 550 and the 1-system optical fiber line
560, by using the optical signal of the other optical fiber line.
Further, the signal can be output to the optical receiver 540, by
separating the respective signals.
[0288] In the configuration of the hitless protection switching
apparatus 530 in FIG. 31, the hitless protection switching
apparatus 530 may be formed so that the signals output from the
amplifiers 5300a and 5300b are directly input into the light
variable delay units 5303a and 5303b, by removing the dispersion
compensating units 5301a and 5301b, and the amplifiers 5302a and
5302b.
(9) Combining Optical Delay Process and Buffering Process of Signal
to Memory
[0289] According to the first embodiment, the delay difference
between the 0-system signal and the 1-system signal is absorbed, by
using the multi-wavelength light variable delay units 305a and 305b
shown in FIG. 1. However, the delay difference may be absorbed,
after the multi-wavelength light variable delay units 305a and 305b
perform an optical delay process, by converting an optical signal
to an electric signal and by buffering the electric signal to a
memory.
[0290] A technology of absorbing a delay difference by buffering a
signal to a memory is conventionally known. However, by combining
with an optical delay process explained in the first embodiment,
the memory capacities required for adjusting the delay difference
can be reduced considerably.
[0291] FIG. 32 is a functional configuration diagram of a route
switching system that includes multi-wavelength light variable
delay units 6205a and 6205b, and memories 6211a and 6211b. This
hitless protection switching system includes a WDM transmission
apparatus 600, a branching apparatus 610, a route switching
apparatus 620, and a receiving apparatus 630. The branching
apparatus 610 and the route switching apparatus 620 are connected
via a 0-system optical fiber line 640 and a 1-system optical fiber
line 650.
[0292] The WDM transmission apparatus 600 is an apparatus that
multiplexes optical signals transmitted from an optical transmitter
(not shown) using the wavelength division multiplexing, and
transmits the multiplexed optical signals with different
wavelengths.
[0293] The branching apparatus 610 is an apparatus that multiplexes
an optical signal for delay adjustment to the optical signal
received from the WDM transmission apparatus 600, and then branches
and transmits the resulting optical signal. The branching apparatus
610 includes a delay adjustment light source 6100, a wavelength
multiplexing unit 6101, and a branching unit 6102. The delay
adjustment light source 6100, the wavelength multiplexing unit
6101, and the branching unit 6102 are the same as the delay
adjustment light source 200, the wavelength multiplexing unit 201,
and the branching unit 202 explained in FIG. 1.
[0294] The receiving apparatus 630 is an apparatus that, when an
optical signal is converted into an electric signal by the route
switching apparatus 620, receives the electric signal.
[0295] The route switching apparatus 620 is an apparatus that, when
a signal interruption occurs to one of the 0-system optical fiber
line 640 and the 1-system optical fiber line 650, switches the
communication route of the signal to the other of the 0-system
optical fiber line 640 and the 1-system optical fiber line 650
without signal interruption.
[0296] The route switching apparatus 620 converts the wavelength of
the optical signals that have passed through the 0-system optical
fiber line 640 and the 1-system optical fiber line 650, and
continuously adjusts the transmission delay difference of the
optical signals, by making the optical signal pass through an
optical fiber that has wavelength-dependent transmission speed of
an optical signal. The route switching apparatus 620, after
adjusting the transmission delay difference of the optical signal,
compensates the waveform distortion of the optical signal while
maintaining the transmission delay difference.
[0297] The route switching apparatus 620 also converts the optical
signal whose delay difference is adjusted to an electric signal,
and buffers to a memory. When a signal interruption occurs, the
route switching apparatus 620 performs a process of reading out a
signal from the memory of one of the 0-system and the 1-system
without signal interruption.
[0298] The route switching apparatus 620 includes amplifiers 6200a
and 6200b, dispersion compensating units 6201a and 6201b,
amplifiers 6202a and 6202b, low-dispersion fiber buffers 6203a and
6203b, amplifiers 6204a and 6204b, multi-wavelength light variable
delay units 6205a and 6205b, delay amount constant dispersion
compensating units 6206a and 6206b, amplifiers 6207a and 6207b,
variable attenuators 6208a and 6208b, wavelength separating units
6209a and 6209b, optical/electrical converters 6210a and 6210b,
memories 6211a and 6211b, a light interruption detecting unit 6212,
a switching unit 6213, and a controller 6214.
[0299] The amplifiers 6200a and 6200b, the dispersion compensating
units 6201a and 6201b, the amplifiers 6202a and 6202b, the
low-dispersion fiber buffers 6203a and 6203b, the amplifiers 6204a
and 6204b, the multi-wavelength light variable delay units 6205a
and 6205b, the delay amount constant dispersion compensating units
6206a and 6206b, the amplifiers 6207a and 6207b, the variable
attenuators 6208a and 6208b, and the wavelength separating units
6209a and 6209b are functioning units that are the same as the
amplifiers 300a and 300b, the dispersion compensating units 301a
and 301b, the amplifiers 302a and 302b, the low-dispersion fiber
buffers 303a and 303b, the amplifiers 304a and 304b, the
multi-wavelength light variable delay units 305a and 305b, the
delay amount constant dispersion compensating units 306a and 306b,
the amplifiers 307a and 307b, the former-stage variable attenuators
308a and 308b, and the wavelength separating units 309a and 309b
shown in FIG. 1, respectively.
[0300] The optical/electrical converters 6210a and 6210b are
processing units that convert optical signals received respectively
from the wavelength separating units 6209a and 6209b to electric
signals. The memories 6211a and 6211b are memories that store
therein the electric signals output respectively from the
optical/electrical converters 6210a and 6211b.
[0301] The light interruption detecting unit 6212 is a processing
unit that detects whether a signal interruption has occurred in one
of the 0-system optical fiber line 640 and the 1-system optical
fiber line 650, by monitoring the signal input to the amplifiers
6200a and 6200b. If a signal interruption is detected, the light
interruption detecting unit 6212 notifies the switching unit 6213
of the signal interruption.
[0302] The switching unit 6213 is a processing unit that reads out
the signal stored in one of the memories 6211a or 6211b
respectively of the 0-system and the 1-system. The switching unit
6213, when a signal interruption occurs in one of the 0-system and
the 1-system, reads out the signal from one of the memories 6211a
and 6211b of the other of the 0-system and the 1-system without
signal interruption.
[0303] The controller 6214 performs the same process as the
controller 314 explained in FIG. 18. The controller 6214 controls
the one-step wavelength converters 3010 and 3012, the two-step
wavelength converters 3013 and 3015, the delay amount constant
dispersion compensating units 306a and 306b, and the former-stage
variable attenuators 308a and 308b. The controller 6214 then
adjusts a delay difference and an intensity level difference
between the 0-system signal and the 1-system signal.
[0304] When recovered from the signal interruption, and when the
switching unit 6213 reswitches the memories 6211a and 6211b from
which the signal is read out, the delay difference is to be
absorbed as much as possible, by executing the processes from step
S301 to step S307 shown in FIG. 21.
[0305] In this manner, the optical signals of the 0-system and the
1-system, in which waveform degradation is compensated, are stored
in the memories 6211a and 6211b. When a signal interruption occurs
to one of the routes of the 0-system and the 1-system, the signal
stored in one of the memories 6211a and 6211b of the route without
signal interruption is read out. As a result, the storage
capacities of the memories 6211a and 6211b, for example, can be
reduced, and reliability of the signal transmission can be
improved.
[0306] When the signal interruption occurred in one of the 0-system
route and the 1-system route is absorbed, and the signal stored in
the memories 6211a and 6211b at the route where the signal
interruption is absorbed is read out, the multi-wavelength light
variable delay units 6205a and 6205b readjust the transmission
delay difference of the optical signal. As a result, a recovery
from the signal interruption can be performed efficiently.
(10) Others
[0307] The present invention can be implemented in various
modifications within the spirit and scope of the appended claims,
other than the above-described embodiments.
[0308] For example, among the respective processes explained in the
embodiments, all or a part of the process explained as being
performed automatically may be performed manually. Or, all or a
part of the process explained as being performed manually may be
performed automatically by a known method.
[0309] The information that includes the process procedure, the
controlling procedure, specific names, and various data and
parameters shown in the description and the drawings may be changed
in any way, unless otherwise specified.
[0310] The respective constituent elements of the respective
apparatuses shown in the drawings are functional concepts, and the
same physical configuration as in the drawings is not necessarily
required. In other words, the specific mode of dispersion and
integration of the respective apparatuses is not limited to the
ones shown in the drawings, but all or a part thereof may be
functionally or physically dispersed or integrated in any unit,
depending on various loads and usage states.
[0311] All or any part of the respective processing functions
performed at the respective apparatuses can be realized by the CPU
and a program that is analyzed and executed by the CPU, or may be
realized as hardware by a wired logic.
[0312] According to one exemplary embodiment of the present
invention, a wavelength of an optical signal is converted, a
transmission delay difference between the optical signals of each
route is adjusted by making the optical signal with a converted
wavelength pass through a waveguide in which a transmission delay
of the optical signal changes continuously depending on its
wavelength, and degradation of a waveform of the optical signal is
compensated while maintaining the adjusted transmission delay
difference. As a result, the transmission delay difference of the
optical signal that is transmitted via a plurality of redundant
routes can be advantageously and continuously adjusted, while
preventing degradation of the optical signal.
[0313] According to one exemplary embodiment of the invention, when
the wavelength of the optical signal is converted, the wavelength
of the optical signal is reconverted into a predetermined
wavelength. As a result, the wavelength of the optical signal with
a converted wavelength can be advantageously returned to the
original wavelength.
[0314] According to one exemplary embodiment of the invention, the
transmission delay difference of the optical signal is adjusted, by
switching a plurality of waveguides with different lengths and
discretely changing the transmission delay of the optical signal.
Further, the wavelength of the optical signal is converted, and the
transmission delay difference between the optical signals of each
route is adjusted, by making the optical signal with a converted
wavelength pass through the waveguide in which the transmission
delay of the optical signal changes continuously depending on the
wavelength. As a result, the transmission delay difference of the
optical signal can be advantageously and broadly adjusted by
discretely changing the transmission delay, and the transmission
delay difference of the optical signal can be advantageously
refined, by continuously changing the transmission delay
afterwards.
[0315] According to one exemplary embodiment of the invention, the
transmission delay difference between a plurality of optical
signals which are transmitted via redundant routes and multiplexed
by the wavelength division multiplexing is adjusted by signals with
the same wavelength. As a result, the transmission delay difference
between the multiplexed signals with the same wavelength can be
advantageously adjusted, by using the wavelength division
multiplexing.
[0316] According to one exemplary embodiment of the invention, the
optical signal whose delay difference detecting signal used for
detecting the transmission delay difference of the optical signal
is multiplexed is received, and the transmission delay difference
between the optical signals is adjusted by referring to the delay
difference detecting signal. As a result, the transmission delay
difference between the optical signals can be advantageously
adjusted, by multiplexing the delay difference detecting signal to
the optical signal.
[0317] According to one exemplary embodiment of the invention, the
optical signal of each route is combined and output, and when a
signal interruption is detected, an intensity level of the optical
signal of each route, in which degradation of the waveform is
compensated, is adjusted so that the intensity level of the optical
signal to be output is of a predetermined level. As a result, a
fluctuation of the intensity level of the optical signal can be
advantageously suppressed, when a signal interruption occurs.
[0318] According to one exemplary embodiment of the invention, the
transmission delay difference of the optical signal is adjusted, by
switching the waveguides with different lengths and discretely
changing the transmission delay of the optical signal. The
wavelength of the optical signal is converted, and the transmission
delay difference of the optical signal is adjusted, by making the
optical signal with a converted wavelength pass through the
waveguide that generates the transmission delay depending on the
wavelength of the optical signal. Further, a signal interruption
that is generated to the signal before the optical signal is input
is detected at the waveguides that discretely change the
transmission delay of the optical signal. As a result, a
fluctuation of the intensity level of the optical signal can be
advantageously suppressed, by detecting the signal interruption
early, before the transmission delay difference of the signal is
adjusted.
[0319] According to one exemplary embodiment of the invention, when
a signal interruption is detected, the intensity level of the
optical signal of each route is adjusted, before the optical signal
stops reaching. As a result, a transmission of the optical signals
can be advantageously prevented from a momentary interruption.
[0320] According to one exemplary embodiment of the invention, the
optical signal of each route is combined and output, the intensity
level of the optical signal being output is monitored, and the
intensity level of the optical signal of each route, in which
degradation of the waveform of the optical signal is compensated,
is adjusted so as that intensity level becomes a predetermined
level. As a result, a fluctuation of the intensity level of the
optical signal can be advantageously suppressed, by monitoring the
intensity level of the optical signal being output.
[0321] According to one exemplary embodiment of the invention, the
optical signals of the routes, in which degradation of the waveform
is compensated, are stored electrically, and when a signal
interruption occurs to one of the routes, the signal stored in the
route without signal interruption is read out. As a result,
reliability of the signal transmission can be advantageously
improved, while storage capacities such as a memory that stores
therein a signal electrically can be reduced, by combining an
adjustment process of the transmission delay difference of the
optical signal and an electrical storage process.
[0322] According to one exemplary embodiment of the invention, when
the signal interruption occurred in one route is absorbed, and the
signal stored in this route in which the signal interruption is
absorbed is read out, the transmission delay difference of the
optical signal is readjusted. As a result, recovery from the signal
interruption can be advantageously performed.
[0323] According to one exemplary embodiment of the invention, a
communication error signal that is generated when a delay
difference of the optical signals of the routes is detected, and
the transmission delay difference between the optical signals of
each route is adjusted until the communication error signal is not
detected any more. As a result, an adjustment of the transmission
delay difference of the optical signal can be advantageously and
easily performed, by detecting the communication error signal
instead of detecting the delay difference itself.
[0324] Although the invention has been described with respect to
specific embodiments for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art that fairly fall within the
basic teaching herein set forth.
* * * * *