U.S. patent application number 12/114688 was filed with the patent office on 2008-12-04 for chirp measurement method, chirp measurement apparatus and their application.
Invention is credited to Tetsuro Inui, Kunihiko Mori, Kohichi Robert Tamura.
Application Number | 20080296481 12/114688 |
Document ID | / |
Family ID | 26624317 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080296481 |
Kind Code |
A1 |
Inui; Tetsuro ; et
al. |
December 4, 2008 |
CHIRP MEASUREMENT METHOD, CHIRP MEASUREMENT APPARATUS AND THEIR
APPLICATION
Abstract
A chirp measurement apparatus includes a splitting section for
splitting input signal light to two paths; a first dispersion
medium with a total dispersion amount of +D (.noteq.0) at a used
wavelength, and a second dispersion medium with a total dispersion
amount of -D (.noteq.0) at the used wavelength; first and second
nonlinear photo-detecting sections for receiving the signal light
beams passing through the first and second dispersion media, and
for outputting electric signals with the intensities proportional
to nth power of the intensities of the signal light beams, where n
is greater than one; and a difference detecting section for
computing a difference between the electric signals output from the
first and second nonlinear photo-detecting sections, and for
outputting a differential signal corresponding to the difference as
a chirp signal of the input signal light.
Inventors: |
Inui; Tetsuro;
(Yokohama-shi, JP) ; Mori; Kunihiko;
(Yokosuka-shi, JP) ; Tamura; Kohichi Robert;
(Yokosuka-shi, JP) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
26624317 |
Appl. No.: |
12/114688 |
Filed: |
May 2, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11232963 |
Sep 23, 2005 |
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12114688 |
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10284286 |
Oct 31, 2002 |
6958467 |
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11232963 |
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Current U.S.
Class: |
250/227.23 |
Current CPC
Class: |
H04B 10/2519
20130101 |
Class at
Publication: |
250/227.23 |
International
Class: |
G01N 21/25 20060101
G01N021/25 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2001 |
JP |
2001-337865 |
Jun 5, 2002 |
JP |
2002-164437 |
Claims
1-5. (canceled)
6. A chirp measurement apparatus, comprising: splitting means for
splitting input signal light to two paths; a first dispersion
medium with a total dispersion amount of +D (.noteq.0) at a
wavelength of the input signal light, and first nonlinear
photo-detecting means for receiving the signal light passing
through the first dispersion medium, and for outputting an electric
signal with the intensity proportional to nth power of the
intensity of the signal light, where n is greater than one, said
first dispersion medium and said first nonlinear photo-detecting
means being placed on a first path of the two paths; a second
dispersion medium with a total dispersion amount of -D (.noteq.0)
at the wavelength of the input signal light, and second nonlinear
photo-detecting means for receiving the signal light passing
through the second dispersion medium, and for outputting an
electric signal with the intensity proportional to nth power of the
intensity of the signal light, where n is greater than one, said
second dispersion medium and said second nonlinear photo-detecting
means being placed on a second path of the two paths; difference
detecting means for obtaining a difference between the electric
signals output from said first nonlinear photo-detecting means and
said second nonlinear photo-detecting means, and for outputting a
differential signal corresponding to the difference as a chirp
signal of the input signal light; and in said two paths, first and
second optical signal intensity adjusting means for adjusting, when
the input signal light unaffected by dispersion is input, signal
light intensities supplied to said first and second nonlinear
photo-detecting means such that intensities of the electric signals
supplied from the two paths to said difference detecting means
become equal.
7. The chirp measurement apparatus as claimed in claim 6, further
comprising control means for controlling the differential signal
output from said difference detecting means such that the
differential signal becomes minimum by feeding the differential
signal back to at least one of said first and second optical signal
adjusting means.
8. A chirp measurement apparatus, comprising: splitting means for
splitting input signal light to two paths; a first dispersion
medium with a total dispersion amount of +D (.noteq.0) at a
wavelength of the input signal light, and first nonlinear
photo-detecting means for receiving the signal light passing
through the first dispersion medium, and for outputting an electric
signal with the intensity proportional to nth power of the
intensity of the signal light, where n is greater than one, said
first dispersion medium and said first nonlinear photo-detecting
means being placed on a first path of the two paths; a second
dispersion medium with a total dispersion amount of -D (.noteq.0)
at the wavelength of the input signal light, and second nonlinear
photo-detecting means for receiving the signal light passing
through the second dispersion medium, and for outputting an
electric signal with the intensity proportional to nth power of the
intensity of the signal light, where n is greater than one, said
second dispersion medium and said second nonlinear photo-detecting
means being placed on a second path of the two paths; difference
detecting means for obtaining a difference between the electric
signals output from said first nonlinear photo-detecting means and
said second nonlinear photo-detecting means, and for outputting a
differential signal corresponding to the difference as a chirp
signal of the input signal light; and in said two paths, first and
second electric signal intensity adjusting means for adjusting,
when the input signal light unaffected by dispersion is input,
signal light intensities supplied to said first and second
nonlinear photo-detecting means such that intensities of the
electric signals supplied from the two paths to said difference
detecting means become equal.
9. The chirp measurement apparatus as claimed in claim 8, further
comprising control means for controlling the differential signal
output from said difference detecting means such that the
differential signal becomes minimum by feeding the differential
signal back to at least one of said first and second electric
signal adjusting means.
10-18. (canceled)
19. A chirp measurement method, comprising: a first step of
splitting input signal light to two paths; a second step of
supplying signal light beams traveling through the two paths to
first and second nonlinear photo-detecting means through first and
second dispersion media with total dispersion amounts of +D
(.noteq.0) and -D (.noteq.0) at the wavelength of the input signal
light beams, respectively, to convert the individual signal light
beams traveling through the two paths to electric signals with
intensities proportional to nth power of input optical intensities,
where n is greater than one; and a third step of outputting a
differential signal corresponding to a difference between the
electric signals of the two paths as a chirp signal of the input
signal light a fourth step of inputting the input signal light
unaffected by the dispersion; a fourth step of inputting the input
signal light unaffected by the dispersion; and a fifth step of
adjusting signal light intensities of the two paths or electric
signal intensities output from said first and second nonlinear
photo-detecting means such that the differential signal becomes
minimum by using first and second signal intensity adjusting means
provided in the two paths.
20. (canceled)
Description
[0001] This application is based on Japanese Patent Application No.
2001-337865 filed Nov. 2, 2001 and No. 2002-164437 filed Jun. 5,
2002, the content of which are incorporated hereinto by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a chirp measurement
apparatus and its application for detecting wavelength dispersion
and its fluctuations in an optical fiber in a terminal or in a
linear repeater and regenerator of a ultrafast, large capacity
optical communication system. In addition, the present invention
relates to a chirp measurement method for carrying out calibration
using input signal light unaffected by the wavelength dispersion of
the optical fiber.
[0004] 2. Description of the Related Art
[0005] Optical fiber transmission lines have dispersion
fluctuations due to environmental changes such temperature
variations and pressure application. Accordingly, it is necessary
for ultrafast large capacity optical communication systems to
employ an adaptive dispersion equalization technique for
automatically detecting the dispersion fluctuations of the optical
fiber transmission lines to carry out equalization. As conventional
chirp measurement methods for automatically detecting the
dispersion fluctuations, the following methods are known.
[0006] An adaptive dispersion equalization scheme utilizing a
wavelength tunable laser: It monitors the intensity of a 40 GHz
electric clock, and dithers the wavelength of the signal light to
set it at the optimum wavelength (G. Ishikawa et al., ECOC'98, p.
519, 1998). It has a problem in that the wavelength of the signal
light varies.
[0007] An adaptive dispersion equalization scheme utilizing a VIPA
(Virtually Imaged Phased Array) tunable dispersion equalizer: It
employs a method of dithering the dispersion of an equalizer to
detect the fluctuations in the dispersion (H. Ooi et al.,
OECC'2001, PD5, 2001). It dithers the dispersion value in a range
of .+-.3 ps/nm, and has a problem in that it cannot avoid
characteristic degradation in the long run because it includes an
movable optical section.
[0008] An adaptive dispersion equalizer using a fiber grating: It
employs a method of detecting the dispersion fluctuations in a
single mode fiber transmission line by PM-AM conversion (K. M. Feng
et al., IEEE Photon. Technol. Lett., vol. 11, no. 3, p. 373, 1999).
The single mode fiber always has anomalous dispersion in 1.5 .mu.m
band, and can utilize the PM-AM conversion because the sign of the
optical fiber with negative dispersion (-D ps/nm) is disposed as
the path 2. The signal light beams passing through the optical
fibers constituting the two paths are each converted into an
electric signal by a photodiode (PD), and supplied to an RF
detector for detecting the level of the clock signal via a bandpass
filter (BPF). The levels of the clock signals of the two paths are
compared by a differential amplifier, which outputs a differential
signal.
[0009] The optical fiber transmission line has dispersion
fluctuations due to environmental changes, and the dispersion
fluctuations make the level of the clock signal (output voltage) of
one of the two paths greater than that of the other path as
illustrated in FIG. 2A. In this case, controlling a tunable
dispersion equalizer (not shown) inserted into the optical fiber
transmission line can minimize the differential signal so that the
levels of the clock signals of the two paths become equal as
illustrated in FIG. 2B, thereby being able to equalize the
dispersion fluctuations at high accuracy.
[0010] Thus, the conventional configuration as shown in FIG. 1 can
detect the dispersion fluctuations with the fluctuation direction
of the dispersion of the optical fiber transmission line by using
the differential signal between the levels of the clock signals of
the two paths. Accordingly, it is applicable to various types of
optical fiber transmission lines.
[0011] However, the chirp measurement apparatus with the
conventional configuration must determine the passband of its clock
extraction circuit (bandpass filter and RF detector) in accordance
with the bit rate of the transmission system. This is because it
establishes synchronization by extracting the clock signal. As a
result, a new problem arises that it is difficult for the
conventional system to flexibly cope with considerable changes in
the bit rate of the optical signal to be measured.
SUMMARY OF THE INVENTION
[0012] It is therefore an object of the present invention to
provide a chirp measurement apparatus and a chirp measurement
method capable of detecting the dispersion fluctuations of the
optical fiber transmission line independently of the transmission
optical fiber or bit rate, or without dithering the wavelength of
the signal light or the dispersion of the tunable dispersion
equalizer, or without using other monitor light, or without
extracting the clock signal, by utilizing the property that when
the average optical power is constant of the input light to the
nonlinear photo-detector such as a two-photon absorption device,
the average output of the nonlinear photo-detector increases as the
pulse width narrows.
[0013] The central features the present invention are summarized as
follows.
[0014] According to a first aspect of the present invention, there
is provided a chirp measurement apparatus comprising: splitting
means for splitting input signal light to two paths; a first
dispersion medium with a total dispersion amount of +D (.noteq.0)
at a wavelength of the input signal light, and first nonlinear
photo-detecting means for receiving the signal light passing
through the first dispersion medium, and for outputting an electric
signal with the intensity proportional to nth power of the
intensity of the signal light, where n is greater than one, the
first dispersion medium and the first nonlinear photo-detecting
means being placed on a first path of the two paths; a second
dispersion medium with a total dispersion amount of -D (.noteq.0)
at the wavelength of the input signal light, and second nonlinear
photo-detecting means for receiving the signal light passing
through the second dispersion medium, and for outputting an
electric signal with the intensity proportional to nth power of the
intensity of the signal light, where n is greater than one, the
second dispersion medium and the second nonlinear photo-detecting
means being placed on a second path of the two paths; and
difference detecting means for obtaining a difference between the
electric signals output from the first nonlinear photo-detecting
means and the second nonlinear photo-detecting means, and for
outputting a differential signal corresponding to the difference as
a chirp signal of the input signal light.
[0015] According to a second aspect of the present invention, there
is provided a chirp measurement apparatus comprising: splitting
means for splitting input signal light to two paths; a first
dispersion medium with a total dispersion amount of +D (.noteq.0)
at the wavelength of the input signal light, and first polarization
beam splitting means for splitting the signal light into polarized
waves orthogonal to each other, the first dispersion medium and the
first polarization beam splitting means being placed in a first
path of the two paths; a second dispersion medium with a total
dispersion amount of -D (.noteq.0) at the wavelength of the input
signal light, and second polarization beam splitting means for
splitting the signal light into polarized waves orthogonal to each
other, the second dispersion medium and the second polarization
beam splitting means being placed in a second path of the two
paths; a first pair of nonlinear photo-detecting means for
receiving the polarized waves split by the first polarization beam
splitting means, and for outputting electric signals with
intensities proportional to nth power of the optical intensities of
the polarized waves, where n is an integer greater than one; a
second pair of nonlinear photo-detecting means for receiving the
polarized waves split by the second polarization beam splitting
means, and for outputting electric signals with intensities
proportional to nth power of the optical intensities of the
polarized waves, where n is an integer greater than one; and
processing means for computing chirp of the input signal light from
the electric signals output from the first pair and second pair of
the nonlinear photo-detecting means, and for outputting it as a
chirp signal.
[0016] According to a third aspect of the present invention, there
is provided a chirp measurement apparatus comprising: a dispersion
medium with a total dispersion amount of +D (.noteq.0) at the
wavelength of the input signal light; and nonlinear photo-detecting
means for receiving signal light passing through the dispersion
medium, for outputting an electric signal with intensity
proportional to nth power of the optical intensity of the signal
light, and for outputting the electric signal as a chirp signal of
the input signal light.
[0017] According to a fourth aspect of the present invention, there
is provided a chirp measurement method comprising: a first step of
splitting input signal light to two paths; a second step of
supplying signal light beams traveling through the two paths to
first and second nonlinear photo-detecting means through first and
second dispersion media with total dispersion amounts of +D
(.noteq.0) and -D (.noteq.0) at the wavelength of the input signal
light beams, respectively, to convert the individual signal light
beams traveling through the two paths to electric signals with
intensities proportional to nth power of input optical intensities,
where n is greater than one; and a third step of outputting a
differential signal corresponding to a difference between the
electric signals of the two paths as a chirp signal of the input
signal light.
[0018] According to a fifth aspect of the present invention, there
is provided a dispersion compensating apparatus comprising the
chirp measurement apparatus in accordance with the present
invention, and tunable dispersion equalization means for canceling
the chirp of the input signal light measured by the chirp
measurement apparatus.
[0019] The chirp measurement apparatus and chirp measurement method
in accordance with the present invention described above can detect
the dispersion fluctuations in the optical fiber transmission line
independently of the transmission optical fiber and bit rate, and
without dithering the wavelength of the signal light or the
dispersion of the tunable dispersion equalizer, or without using
other monitoring light, or without extracting the clock signal. In
particular, the configuration using the differential signal can
detect the dispersion fluctuations in the optical fiber
transmission line with the sign of the fluctuations. In addition,
being independent of the bit rate, the chirp measurement apparatus
in accordance with the present invention is applicable to the
adaptive dispersion equalization of optical transmission systems
with various bit rates, making it possible to reduce the number of
components and the size and cost of the systems.
[0020] Furthermore, the dispersion compensating apparatus in
accordance with the present invention is configured such that it
utilizes the chirp measurement apparatus in accordance with the
present invention to measure the chirp of the optical signal output
from the optical fiber transmission line, and controls the tunable
dispersion equalizer to cancel out the chirp. Accordingly, it is
independent of the bit rate of the transmission system to which it
is applied, making it possible to apply it to 10 Gbit/s to 40
Gbit/s optical transmission systems and further to 160 Gbit/s
optical transmission systems.
[0021] The above and other objects, effects, features and
advantages of the present invention will become more apparent from
the following description of embodiments thereof taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a block diagram showing a configuration of a
conventional dispersion detection apparatus;
[0023] FIGS. 2A and 2B are diagrams illustrating the variations in
the output voltages of the two paths when carrying out the adaptive
dispersion equalization, wherein FIG. 2A illustrates the output
voltages before the adaptive equalization, and FIG. 2B illustrates
them after the adaptive equalization;
[0024] FIG. 3 is a block diagram showing a basic configuration of a
chirp measurement apparatus in accordance with the present
invention;
[0025] FIG. 4 is a graph illustrating calculation results of a
differential signal;
[0026] FIG. 5 is a block diagram showing a first configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0027] FIG. 6 is a graph illustrating the output voltage of the
differential amplifier versus a measured chirp parameter;
[0028] FIG. 7 is a block diagram showing a second configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0029] FIG. 8 is a block diagram showing a fifth configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0030] FIG. 9 is a block diagram showing a sixth configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0031] FIG. 10 is a block diagram showing a seventh configuration
of the chirp measurement apparatus in accordance with the present
invention;
[0032] FIG. 11 is a block diagram showing an eighth configuration
of the chirp measurement apparatus in accordance with the present
invention;
[0033] FIG. 12 is a block diagram showing a ninth configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0034] FIG. 13 is a block diagram showing a tenth configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0035] FIG. 14 is a block diagram showing an 11th configuration of
the chirp measurement apparatus in accordance with the present
invention;
[0036] FIG. 15 is a block diagram showing a configuration of a
dispersion compensating apparatus in accordance with the present
invention;
[0037] FIGS. 16A and 16B are block diagrams showing positional
relationships between the chirp measurement apparatus and tunable
dispersion equalizer on the optical fiber transmission line in the
dispersion compensating apparatus in accordance with the present
invention;
[0038] FIG. 17 is a graph illustrating an adaptive dispersion
equalization method using the chirp measurement apparatus in
accordance with the present invention; and
[0039] FIG. 18 is a block diagram showing a 12th configuration of
the chirp measurement apparatus in accordance with the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0040] The embodiments in accordance with the present invention
will now be described with reference to the accompanying
drawings.
[0041] FIG. 3 is a block diagram showing a basic configuration of a
chirp measurement apparatus in accordance with the present
invention. The chirp measurement apparatus in accordance with the
present invention utilizes the property that when the average
optical power is constant of the input light to the nonlinear
photo-detector such as a two-photon absorption device, the average
output of the nonlinear photo-detector increases as the pulse width
narrows.
[0042] In FIG. 3, a splitting section 11 divides part of the signal
light (input signal light) traveling through an optical fiber
transmission line 10, a transmission line of the input signal
light, and further splits it into two paths. FIG. 3 illustrates the
case where the splitting section 11 splits the signal light into
two equal parts in terms of power. A first path of the two paths is
provided with a dispersion medium 12-1 and a nonlinear
photo-detector 13-1. The dispersion medium 12-1 has a total
dispersion amount of +D (.noteq.0) at the used wavelength. The
nonlinear photo-detector 13-1 outputs an electric signal (voltage)
with the intensity proportional to the nth power of the optical
intensity of the signal light passing through the dispersion
medium, where n is greater than one. Likewise, a second path of the
two paths is provided with a dispersion medium 12-2 and a nonlinear
photo-detector 13-2. The dispersion medium 12-2 has a total
dispersion amount of -D (.noteq.0) at the used wavelength. The
nonlinear photo-detector 13-2 outputs an electric signal (voltage)
with the intensity proportional to the nth power of the optical
intensity of the signal light passing through the dispersion
medium, where n is greater than one.
[0043] When the input signal light passing through the optical
fiber transmission line 10 undergoes the dispersion (that is, when
the chirp is present), the pulse width of the first path (path 1 of
FIG. 3) narrows and that of the second path (path 2 of FIG. 3)
broaden because of the dispersion media 12-1 and 12-2. In other
words, the peak power of the first path increases and that of the
second path decreases. The nonlinear photo-detectors 13-1 and 13-2
output electric signals (voltages) corresponding to the peak
powers. For example, assume that the nonlinear photo-detectors each
output an electric signal proportional to the square of the optical
intensity, that the signal light pulse is a Gaussian pulse with
average power E and with linear chirp, that the chirp parameter is
C, and the bandwidth of the signal light is .DELTA..omega., and
that the product of the second order derivative of the propagation
constant .beta. and the length of the dispersion medium is B.sub.2,
then the output voltage V(C, .DELTA..omega., B.sub.2) is given by
the following expression.
V(C,.DELTA..omega.,B.sub.2).about.E.sup.2[(1+C.sup.2)/{(1+B.sub.2.DELTA.-
.omega..sup.2C+C.sup.2).sup.2+(B.sub.2.DELTA..omega..sup.2).sup.2}].sup.1/-
2
When the input signal light pulse is chirping under the influence
of the dispersion, the pulse width narrows on the first path and
hence its peak power increases. Thus, the output voltage of the
nonlinear photo-detector 13-1 of the first path increases, whereas
that of the nonlinear photo-detector 13-2 of the second path
decreases. A difference detector 14 detects the difference (voltage
difference) between the electric signals output from the nonlinear
photo-detectors 13-1 and 13-2 of the two paths.
[0044] FIG. 4 is a graph illustrating the calculation results of
the differential signal (V(C, .DELTA..omega., -B.sub.2)-V(C,
.DELTA..omega., B.sub.2)). As illustrated in FIG. 4, the voltage
difference varies monotonically in the range from
-B.sub.2.DELTA..omega..sup.2 to B.sub.2.DELTA..omega..sup.2, and
linearly around C=0. Accordingly, the differential signal can be
used as the dispersion detection signal of the optical fiber
transmission line.
[0045] As the nonlinear photo-detectors 13-1 and 13-2, a nonlinear
photo-detecting device with two-photon absorption can be used.
Alternatively, it is possible to use a nonlinear medium generating
second harmonic light of the input light and a photo-detector for
converting the second harmonic light into an electric signal. In
addition, a filter that passes only the second harmonic light can
be interposed between the nonlinear medium and photo-detector.
[0046] When the input signal light untouched by the dispersion is
input, the dispersion media 12-1 and 12-2 do not cause any
difference between the output peak powers so that the differential
signal is nearly zero. In other words, as long as the signal light
is affected by the dispersion, the differential signal is generated
and the chirp detection is carried out.
[0047] Consider the case where the splitting ratio of the splitting
section 11 is not 1:1, or where even if the splitting ratio is 1:1,
the losses of the two paths after the splitting differ. In such
cases, even if the input signal light unaffected by the dispersion
is supplied, the intensities of the electric signals supplied from
the two paths to the difference detector are unequal, and hence the
differential signal is generated. In this case, to match the
intensities of the electric signals supplied from the two paths to
the difference detector when the input signal light unaffected by
the dispersion of the optical fiber transmission line is input, it
is possible to implement such a configuration that comprises an
optical signal adjusting section for regulating the optical signal
intensities supplied to the nonlinear photo-detectors of the two
paths, or an electric signal adjusting section for regulating the
intensities of the electric signals output from nonlinear
photo-detectors of the two paths. This makes it possible to
compensate for the splitting ratio or the difference between the
losses of the two paths in advance, thereby enabling producing the
differential signal corresponding to the dispersion optical fiber
transmission line accurately.
[0048] It is also possible to implement such a configuration that
feeds differential signal supplied from the difference detector
back to at least one of the optical signal adjusting sections, or
at least one of the electric signal adjusting sections the two
paths comprise, and that regulates the differential signal to
become minimum when the input signal light unaffected by the
dispersion of the optical fiber transmission line is input.
[0049] Alternatively, it is possible to implement such a
configuration that comprises a coupler for dividing part of the
input signal light supplied to the splitting section, a linear
photo-detector for outputting an electric signal V.sub.1
proportional to its optical intensity, and normalization sections
for normalizing electric signals V.sub.21 and V.sub.22 output from
the nonlinear photo-detectors of the two paths to electric signals
V.sub.21/V.sub.1.sup.n and V.sub.22/V.sub.1.sup.n by the electric
signal V.sub.1, and for supplying them to the difference
detector.
[0050] Likewise, it is possible to implement such a configuration
that comprises couplers for dividing parts of the signal light
beams input to the nonlinear photo-detectors, linear
photo-detectors for outputting electric signals V.sub.11 and
V.sub.21 proportional to their optical intensities, and
normalization sections for normalizing the electric signals
V.sub.21 and V.sub.22 output from the nonlinear photo-detectors of
the two paths to electric signals V.sub.21/V.sub.11.sup.n and
V.sub.22/V.sub.12.sup.n by the electric signals V.sub.21 and
V.sub.22, and for supplying them to the difference detector.
[0051] It is also possible to implement such a configuration that
comprises a dispersion medium with the total dispersion amount +D
(.noteq.0) at the used wavelength, and a nonlinear photo-detector
for receiving signal light passing through the dispersion medium
and for outputting the electric signal with the intensity
proportional to the nth power of the optical intensity, where n is
greater than one, wherein the electric signal is output as the
dispersion detection signal of the optical fiber transmission line.
In this case, it is also possible to add a coupler for dividing
part of the signal light to be supplied to the nonlinear
photo-detector, a linear photo-detector for producing an electric
signal V.sub.1 with the intensity proportional to the optical
intensity, and a normalization section for normalizing the electric
signal V.sub.2 output from the nonlinear photo-detector to an
electric signal V.sub.2/V.sub.1.sup.n by the electric signal
V.sub.1.
[0052] Next, embodiments in accordance with the present invention
will be described.
FIRST EMBODIMENT
[0053] FIG. 5 is a block diagram showing a first configuration of
the chirp measurement apparatus in accordance with the present
invention. In FIG. 5, the input signal light traveling through the
optical fiber transmission line 10 undergoes 1/10 power division by
a 90:10 optical coupler 21, followed by further splitting the
signal light to two paths by a 50:50 optical coupler 22 that
divides it to two portions with equal power. A first path (path 1)
of the two paths is provided with a single mode fiber (SMF) 23 with
the total dispersion amount +D (.noteq.0) at the used wavelength as
a dispersion medium, and the second path (path 2) is provided a
dispersion compensating fiber (DCF) 24 with the total dispersion
amount -D (.noteq.0) at the used wavelength as a dispersion
medium.
[0054] In addition, the two paths are provided with Si-APDs
(Silicon Avalanche Photo Diodes) 25-1 and 25-2 that exhibit
two-photon absorption in a 1.5 .mu.m band as nonlinear
photo-detectors for receiving the signal light beams passing
through the individual dispersion media 23 and 24, and for
outputting electric signals with the intensities proportional to
the square of the intensity of the input light beams. Their
response frequency is much lower than the transmission rate so that
they can produce a DC voltage as their output. A differential
amplifier 26 detects the difference between the output voltages of
the Si-APDs 25-1 and 25-2 of the two paths, and outputs the
differential signal (voltage difference).
[0055] FIG. 6 is a graph illustrating the output voltage of the
differential amplifier 26 versus a measured chirp parameter. The
Si-APDs have the response frequency of 10 MHz against the
transmission rate of 10 Gbit/s. In addition, as for the dispersion
media, the single mode fiber has the dispersion value of +16 ps/nm
at the wavelength 1552 nm, and the dispersion compensating fiber
has the dispersion value of -16 ps/nm at the same wavelength. The
signal light is an optical pulse with a bandwidth of 545 GHz fed
from a 10 GHz mode-locked fiber laser. The output voltage exhibits
a linear characteristic for the chirp parameter, which is
consistent with the theoretical calculation result of the
differential signal as illustrated in FIG. 4. In other words, since
the output voltage of the differential amplifier 26 takes the
values corresponding to the magnitude of the dispersion (chirp), it
can be used as the chirp detection signal of the optical pulse
traveling through the optical fiber transmission line 10.
SECOND EMBODIMENT
[0056] FIG. 7 is a block diagram showing a second configuration of
the chirp measurement apparatus in accordance with the present
invention. The second configuration is characterized in that it
employs KH.sub.2PO.sub.4 (KDPs) 27-1 and 27-2, an SHG (second
harmonic generation) crystal, as the nonlinear media for generating
the second harmonic light of the input light in place of the
Si-APDs used as the nonlinear photo-detectors in the first
configuration, and employs photomultipliers 28-1 and 28-2 as the
photo-detectors for converting the second harmonic light to
electric signals.
[0057] The light beams output from the single mode fiber (SMF) 23
and dispersion compensating fiber (DCF) 24 are incident onto the
KDPs 27-1 and 27-2 via focusing lenses not shown. The KDPs 27-1 and
27-2 generate the second harmonic light beams with the output
intensity proportional to the square of the input optical
intensity. The second harmonic light beams (700 nm wavelength band)
are incident onto the photomultipliers 28-1 and 28-2 with a
sensitive wavelength range of 300-820 nm. When the signal light fed
from the optical fiber transmission line 10 undergoes the
dispersion, the output voltages of the photomultipliers 28-1 and
28-2 of the two paths differ, so that the magnitude of the
dispersion (chirp) can be detected by the voltage difference.
THIRD EMBODIMENT
[0058] It is possible for the first and second configurations to
utilize a tunable dispersion equalizer such as a fiber Bragg
grating or VIPA in place of the single mode fiber (SMF) 23 and
dispersion compensating fiber (DCF) 24 used as the dispersion media
disposed in the two paths. To achieve the positive and negative
dispersion values with the same absolute value, it is enough to use
the same chirped fiber Bragg gratings, and to make opposite the
incident direction of the light.
[0059] Alternatively, it is also possible to adjust the dispersion
values of the chirped fiber Bragg gratings by a piezoelectric
transducer or heater in order to set the dispersion values +D
(.noteq.0) and -D (.noteq.0) of the two paths.
FOURTH EMBODIMENT
[0060] In the second configuration, it is possible to interpose
bandpass optical filters, which pass only the second harmonic light
with a frequency 2.omega..sub.1, into the paths from the KDPs 27-1
and 27-2, which receive the signal light with a frequency
.omega..sub.1 and generate the second harmonic light with the
frequency 2.omega..sub.1, to the photomultipliers 28-1 and 28-2.
This makes it possible to eliminate noise other than the second
harmonic light.
FIFTH EMBODIMENT
[0061] FIG. 8 is a block diagram showing a fifth configuration of
the chirp measurement apparatus in accordance with the present
invention. It is assumed in the foregoing configurations that the
signal light is split to the two equal paths in terms of power by
the 50:50 optical coupler 22. In contrast, the present
configuration is an example that can cope with a case where the
input signal light is split into two unequal paths in terms of
power, or into two paths having different losses.
[0062] The configuration has optical amplifiers 29-1 and 29-2
disposed before the single mode fiber (SMF) 23 and dispersion
compensating fiber (DCF) 24 in the two paths. When the input signal
light unaffected by the dispersion of the optical fiber
transmission line 10 (waveforms indicated by broken lines in FIG.
8) is input, the optical amplifiers 29-1 and 29-2 regulate the
optical signal intensities so that the intensities of the electric
signals supplied from the two paths to the differential amplifier
26 become equal. This makes it possible to adjust the intensities
even if the input light is not divided into two equal portions to
the two paths, or the two paths have different losses.
[0063] When using an erbium doped fiber amplifier (EDFA) for the
optical amplifiers 29-1 and 29-2, for example, their optical signal
intensities are controllable by the bias voltages. In addition, the
optical amplifiers 29-1 and 29-2 can carry out the APC (Automatic
Power Control) such that the average input power to the nonlinear
photo-detectors 13-1 and 13-2 become constant. When the splitting
ratio to and the losses of the two paths are each equal, the
optical amplifiers 29-1 and 29-2 of the two paths are unnecessary.
In this case, an optical amplifier (EDFA) can be placed before the
50:50 optical coupler 22 in order to make the average input power
to the nonlinear photo-detectors 13-1 and 13-2 constant.
[0064] It is also possible for at least one of the optical
amplifiers 29-1 and 29-2 to include an additional variable optical
attenuator, or to be replaced by a variable optical attenuator to
control the optical power of the two paths.
[0065] The configuration is designed such that the splitting ratio
to and the losses of the two optical paths are compensated for in
advance before detecting the magnitude of the dispersion of the
optical fiber transmission line 10. In other words, the optical
amplifiers 29-1 and 29-2 control the intensities of the optical
signals such that the intensities of the electric signals supplied
from the two paths to the differential amplifier 26 become equal
when the input signal light unaffected by the dispersion is input
(broken line waveforms in FIG. 8). Accordingly, it is also possible
to use, instead of the optical amplifiers 29-1 and 29-2, electric
signal adjusting sections (for example, electric amplifiers)
installed at the output side of the nonlinear photo-detectors 13-1
and 13-2 in order to equalize the intensities of the electric
signals supplied from the two paths to the differential amplifier
26 by carrying out feedback control of at least one of them by a
control circuit 30.
SIXTH EMBODIMENT
[0066] FIG. 9 is a block diagram showing a sixth configuration of
the chirp measurement apparatus in accordance with the present
invention. The configuration is characterized in that in the fifth
configuration, a control circuit 30 feeds the differential signal
output from the differential amplifier 26 back to at least one of
the optical amplifiers 29-1 and 29-2 of the two paths in order to
regulate the differential signal to a minimum (zero) when the input
signal light (broken line waveforms in FIG. 9) unaffected by the
dispersion of the optical fiber transmission line is input.
[0067] It is also possible for at least one of the optical
amplifiers 29-1 and 29-2 to include an additional variable optical
attenuator, or to be replaced by a variable optical attenuator to
control the optical power of the two paths.
[0068] The configuration is designed such that the splitting ratio
to and the losses of the two optical paths are compensated for in
advance before detecting the magnitude of the dispersion of the
optical fiber transmission line 10. In other words, the optical
amplifiers 29-1 and 29-2 control the intensities of the optical
signals such that the intensities of the electric signals supplied
from the two paths to the differential amplifier 26 become equal
when the input signal light unaffected by the dispersion is input
(broken line waveforms in FIG. 9). Accordingly, it is also possible
to use, instead of the optical amplifiers 29-1 and 29-2, electric
signal adjusting sections (for example, electric amplifiers)
installed at the output side of the nonlinear photo-detectors 13-1
and 13-2 in order to equalize the intensities of the electric
signals supplied from the two paths to the differential amplifier
26 by carrying out feedback control of at least one of them by the
control circuit 30.
SEVENTH EMBODIMENT
[0069] FIG. 10 is a block diagram showing a seventh configuration
of the chirp measurement apparatus in accordance with the present
invention. The configuration is characterized in that in the first
to sixth configurations, an optical coupler 32 is interposed
between the 90:10 optical coupler 21 and the 50:50 optical coupler
22 to divide part of the signal light, and a photodiode 33, which
is a linear photo-detecting device, is provided to detect it.
Assume that the output electric signal is V.sub.1. In addition,
normalization circuits 34-1 and 34-2 for normalizing the output
electric signals V.sub.21 and V.sub.22 of the nonlinear
photo-detectors 13-1 and 13-2 are provided between the nonlinear
photo-detectors 13-1 and 13-2 and the differential amplifier 26.
Here, the nonlinear photo-detectors 13-1 and 13-2 output the
electric signals V.sub.21 and V.sub.22 with the intensities
proportional to the nth power of the signal light beams, and the
normalization circuits 34-1 and 34-2 output the normalized electric
signals V.sub.21/V.sub.1.sup.n and V.sub.22/V.sub.1.sup.n.
Accordingly, the differential amplifier 26 outputs the differential
signal in which fluctuations of the input optical power are
compensated for. Incidentally, when Si-APDs exhibiting the
two-photon absorption are used for the nonlinear photo-detecting
devices, the value n=2.
[0070] Here, the term "normalization" means normalizing the
electric signals V.sub.21 and V.sub.22 output from the nonlinear
photo-detectors 13-1 and 13-2 by the output signal V.sub.1 (or the
nth power thereof) of the photodiode 33. Thus dividing the part of
the signal light to measure and normalize the average optical power
makes it possible to measure the desired chirp amount independently
of the fluctuations in the optical power passing through the
optical fiber transmission line 10.
EIGHTH EMBODIMENT
[0071] FIG. 11 is a block diagram showing an eighth configuration
of the chirp measurement apparatus in accordance with the present
invention. The configuration is characterized in that in the first
to sixth configurations, optical couplers 32-1 and 32-2 are
provided before the nonlinear photo-detectors 13-1 and 13-2 of the
two paths to divide parts of the signal light beams, and
photodiodes 33-1 and 33-2, which are a linear photo-detecting
device, are provided to receive them. Assume that the output
electric signals of the photodiodes 33-1 and 33-2 are V.sub.11 and
V.sub.12. In addition, normalization circuits 34-1 and 34-2 for
normalizing the output electric signals V.sub.21 and V.sub.22 of
the nonlinear photo-detectors 13-1 and 13-2 are provided between
the nonlinear photo-detectors 13-1 and 13-2 and the differential
amplifier 26. Here, the nonlinear photo-detectors 13-1 and 13-2
output the electric signals V.sub.21 and V.sub.22 with the
intensities proportional to the nth power of the signal light
beams, and the normalization circuits 34-1 and 34-2 output the
normalized electric signals V.sub.21/V.sub.11.sup.n and
V.sub.22/V.sub.12.sup.n. Accordingly, the differential amplifier 26
outputs the differential signal in which fluctuations of the input
optical power are compensated for.
[0072] Here, the term "normalization" means normalizing the
electric signals V.sub.21 and V.sub.22 output from the nonlinear
photo-detectors 13-1 and 13-2 by the output signals V.sub.11 and
V.sub.12 (or the nth power thereof) of the photodiodes 33-1 and
33-2. Thus dividing the parts of the input light by the optical
couplers 32-1 and 32-2, and normalizing them by the optical powers
of the light beams input to the individual optical couplers 32-1
and 32-2 makes it possible to measure the desired chirp amount
independently of the fluctuations in the optical power passing
through the optical fiber transmission line 10.
NINTH EMBODIMENT
[0073] FIG. 12 is a block diagram showing a ninth configuration of
the chirp measurement apparatus in accordance with the present
invention. In FIG. 12, the input signal light traveling through the
optical fiber transmission line 10 undergoes 1/10 power division by
the 90:10 optical coupler 21. The bypath includes an optical fiber
35 with the total dispersion amount of D (.noteq.0) at the used
wavelength. In addition, a nonlinear photo-detector 13 is provided
which outputs an electric signal with the intensity proportional to
the nth power of the input optical intensity of the signal passing
through the light optical fiber 35.
[0074] Since the output voltage of the nonlinear photo-detector 13
takes different values depending on the magnitude of the dispersion
(chirp), it can be used as the dispersion detection signal of the
optical fiber transmission line 10. More specifically, it is
possible to measure the chirp by storing in a processing unit (not
shown) a cross-reference table that gives chirp values for the
output voltage of the nonlinear photo-detector 13, which are
measured in advance, and by referring to the cross-reference table
by the output voltage varying depending on the magnitude of the
chirp.
TENTH EMBODIMENT
[0075] FIG. 13 is a block diagram showing a tenth configuration of
the chirp measurement apparatus in accordance with the present
invention. The configuration is characterized in that in the ninth
configuration, an optical coupler 32 is provided before the
nonlinear photo-detector 13 to divide part of the input light, and
a photodiode 33 which is a linear photo-detecting device detects
it. Assume that the electric signal output from the photodiode 33
is V.sub.1. In addition, a normalization circuit 34 is provided at
the output of the nonlinear photo-detector 13 to normalize its
output electric signal V.sub.2. The normalization circuit 34
outputs the normalized electric signal V.sub.2/V.sub.1.sup.n, in
which the fluctuations of the input optical power is compensated
for.
[0076] Since the output electric signal of the normalization
circuit 34 takes different values depending on the magnitude of the
dispersion (chirp), it is possible to measure the chirp by storing
in the processing unit (not shown) the cross-reference table that
gives chirp values for the output electric signal of the
normalization circuit 34, which are measured in advance, and by
referring to the cross-reference table by the output voltage
varying depending on the magnitude of the chirp.
ELEVENTH EMBODIMENT
[0077] FIG. 14 is a block diagram showing an 11th configuration of
the chirp measurement apparatus in accordance with the present
invention. The configuration is characterized in that polarization
beam splitters 15-1 and 15-2 are provided after the dispersion
media 12-1 and 12-2 to split the optical signals fed from the
dispersion media 12-1 and 12-2 to polarized waves orthogonal to
each other. The split polarized waves are supplied to the nonlinear
photo-detectors 13-1a and 13-1b and 13-2a and 13-2b that are
installed for the individual polarized waves so that they output
the electric signals (voltages) with is the intensities
proportional to the nth power of the optical intensities of the
individual signal light beams, where n>1. The electric signals
are supplied to a processing unit 16.
[0078] Since the electric signals supplied to the processing unit
16 take different values depending on the magnitude of the
dispersion (chirp) of the individual polarized waves, it is
possible to measure the chirp and output the measurement result as
the chirp signal 17 by storing in the processing unit 16 the
cross-reference table that gives chirp values for the output
electric signals, which are measured in advance, and by referring
to the cross-reference table by the output voltage varying
depending on the magnitude of the chirp.
(Configurations of Dispersion Compensating Apparatus)
[0079] The dispersion compensating apparatus described in the
present embodiment is characterized in that it uses the chirp
measurement apparatus in accordance with the present invention,
measures the chirp of the optical signal output from the optical
fiber transmission line, and controls the tunable dispersion
equalizer such that the chirp is canceled out.
[0080] FIG. 15 is a block diagram showing a configuration of the
dispersion compensating apparatus in accordance with the present
invention. In FIG. 15, the dispersion compensating apparatus
comprises a tunable dispersion equalizer 40 and the chirp
measurement apparatus 20 in accordance with the present invention.
The chirp measurement apparatus 20 feeds the differential signal
output from the differential amplifier 26 back to the tunable
dispersion equalizer 40 via a control circuit 31.
[0081] Although the chirp measurement apparatus 20 of FIG. 15 is
that of the first configuration, it is obvious that other chirp
measurement apparatuses described in the other embodiments are also
applicable.
[0082] In addition, as for the positional relationship between the
chirp measurement apparatus 20 and the tunable dispersion equalizer
40 on the optical fiber transmission line, although the chirp
measurement apparatus 20 is placed after the tunable dispersion
equalizer 40 as shown in FIG. 15 (see also FIG. 16A), the chirp
measurement apparatus 20 can be placed before the tunable
dispersion equalizer 40 as in the configuration of FIG. 16B,
offering the same advantages.
[0083] The configuration as shown in FIG. 15 employs a nonlinear
chirped fiber Bragg gratings paired type tunable dispersion
equalizer as the tunable dispersion equalizer 40. In the
configuration, the second port of the four-port optical circulator
41 is connected to the longer wavelength side of a chirp fiber
grating 42. In addition, the third port of the four-port optical
circulator 41 is connected to the shorter wavelength side of a
chirp fiber grating 43. Furthermore, the first port of the
four-port optical circulator 41 is used as the input port connected
to the optical fiber transmission line 10, and the fourth port is
used as the output port. The tuning section of the tunable
dispersion equalizer 40 has a structure in which the two chirped
fiber Bragg gratings 42 and 43 are fixed to piezoelectric
transducers 44 adhesively. Applying a voltage to the piezoelectric
transducers to control expansion and contraction from the both
sides enables the reflected wavelength bands of the individual
chirped fiber Bragg gratings 42 and 43 to be shifted in the same
direction, thereby being able to alter the wavelength dispersion
characteristics for the transmitting light.
[0084] It is assumed that the dispersion value variable range of
the tunable dispersion equalizer 40 is -16 ps/nm.about.+16 ps/nm at
the wavelength of the signal light, for example. In addition,
assume that the dispersion value of the single mode fiber (SMF) 23
of the chirp measurement apparatus 20 is +16 ps/nm at the
wavelength of the signal light, and that of the dispersion
compensating fiber (DCF) 24 is -16 ps/nm at the wavelength of the
signal light. When the optical signal pulse from the optical fiber
transmission line 10 is affected by the dispersion, the output
voltages of the Si-APDs 25-1 and 25-2 of the two paths exhibit the
difference corresponding to the dispersion (chirp).
[0085] When the optical signal pulse is affected by the negative
dispersion on the optical fiber transmission line 10, the paths 1
and 2 undergo the following influence before the adaptive
equalization as illustrated in FIG. 2A. In the path 1, the pulse
width narrows and the peak power increases because of the small
effect of the dispersion. In contrast with this, in the path 2, the
pulse width broadens and the peak power reduces because of the
large effect of the dispersion. Accordingly, the output voltage of
the Si-APD 25-1 of the path 1 is high, and the output voltage of
the Si-APD 25-2 of the path 2 is low. The differential amplifier 26
outputs the differential signal corresponding to the difference
between the output voltages. The differential signal is supplied to
control circuit 31 comprising a proportional circuit and an
integral circuit. The control circuit 31 converts the differential
signal to a driving signal for controlling the piezoelectric
transducers 44 of the tunable dispersion equalizer 40. The control
circuit 31 controls the expansion and contraction of the chirp
fiber gratings 42 and 43 such that the differential signal becomes
zero. Thus, the zero dispersion wavelength of the tunable
dispersion equalizer 40 shifts following the dispersion
fluctuations in the optical fiber transmission line 10 as
illustrated in FIG. 17, thereby enabling the adaptive dispersion
equalization (FIG. 2B).
[0086] As described above, the chirp measurement apparatus 20 in
accordance with the present invention utilizes the two-photon
absorption effect rather than the clock extraction method to obtain
the differential signal. As a result, it is applicable to 10
Gbit/s.about.40 Gbit/s optical transmission systems, for example,
and even to 160 Gbit/s optical transmission system independently of
the bit rate of the transmission system.
TWELFTH EMBODIMENT
[0087] FIG. 18 is a block diagram showing a twelfth configuration
of the chirp measurement apparatus in accordance with the present
invention. The twelfth configuration is characterized in that in
the eleventh configuration, optical couplers 32-1 and 32-2 are
provided before the polarization beam splitters 15-1 and 15-2
followed by the nonlinear photo-detectors 13-1a and 13-1b and 13-2a
and 13-2b of the two paths to divide parts of the signal light
beams, and photodiodes 33-1 and 33-2 which are linear
photo-detecting devices provided to receive them. The electric
signals output from the linear photo-detecting devices are supplied
to a processing unit 16.
[0088] Since the electric signals supplied to the processing unit
16 are normalized and take different values depending on the
magnitude of the dispersion (chirp) of the individual polarized
waves, it is possible to measure the chirp and output the
measurement result as the chirp signal 17 by storing in the
processing unit 16 the cross-reference table that gives chirp
values for the output electric signals, which are measured in
advance, and by referring to the cross-reference table by the
output voltage varying depending on the magnitude of the chirp.
[0089] The present invention has been described in detail with
respect to preferred embodiments, and it will now be apparent from
the foregoing to those skilled in the art that changes and
modifications may be made without departing from the invention in
its broader aspect, and it is the intention, therefore, in the
apparent claims to cover all such changes and modifications as fall
within the true spirit of the invention.
* * * * *