SPECIFICATION
OPTICAL SIGNAL PROCESSING DEVICE AND OPTICAL SIGNAL
PROCESSING METHOD
5
TECHNICAL FIELD
[0001]
The present invention relates to an optical signal
processing device that processes an optical signal, and an
10 optical signal processing method.
BACKGROUND ART
[0002]
With the spread of the Internet, an increase in the
15 capacity of an optical communication system has progressed.
For example, in trunk systems, researches for transmitting
signals at a rate exceeding 40 Gbit/s per wavelength have
been performed. When a bit rate per wavelength is made high,
the deterioration of signal quality increases due to a
20 decrease in optical signal-to-noise ratio (OSNR) resistance,
and waveform distortion caused by wavelength dispersion of
a transmission channel, polarization mode dispersion, a
non-linear effect, and the like.
[0003]
25 For this reason, in recent years, digital coherent
reception systems having high OSNR resistance and waveform
distortion resistance have attracted attention (see, for
2
example, Patent Documents 1 and 2). In the digital coherent
reception systems, light intensity information and phase
information are extracted from a received signal, and
demodulation is performed by a digital signal processing
circuit. In the digital coherent reception systems, 5 an
improvement in the OSNR resistance by coherent reception and
a correction of the waveform distortion by the digital signal
processing circuit are realized, and thus even when signals
are transmitted at a rate exceeding 40 Gbit/s, high
10 reliability is achieved.
RELATED DOCUMENT
PATENT DOCUMENT
[0004]
15 [Patent Document 1] Pamphlet of International
Publication WO 09/069814
[Patent Document 2] Japanese Unexamined Patent
Application Publication No. 2010-028795
20 DISCLOSURE OF THE INVENTION
[0005]
In a digital coherent reception device, the common mode
rejection ratio (CMRR) of a photodiode to an optical input
is one of the most important factors having an influence on
25 performance. In addition, in an optical signal reception
device, it is also important to increase a dynamic range.
[0006]
3
An object of the invention is to provide an optical
signal processing device and an optical signal processing
method, which are capable of suppressing an increase in the
CMRR. In addition, another object of the invention is to
provide an optical signal processing device which is capabl5 e
of increasing a dynamic range.
[0007]
According to the invention, there is provided an
optical signal processing device including: a first optical
10 signal generation unit that generates a first optical signal
by causing a received optical signal which is received from
an outside and a local optical signal to interfere with each
other with a first phase difference; a second optical signal
generation unit that generates a second optical signal by
15 causing the received optical signal and the local optical
signal to interfere with each other with a second phase
difference shifted by π from the first phase difference; a
first photoelectric conversion element that converts the
first optical signal into a first electrical signal; a second
20 photoelectric conversion element that converts the second
optical signal into a second electrical signal; a
direct-current component correction unit that reduces a
difference between a magnitude of a direct-current component
of the first electrical signal and a magnitude of a
25 direct-current component of the second electrical signal;
and a differential trans-impedance circuit into which the
first electrical signal and the second electrical signal,
4
which have been corrected by the direct-current component
correction, unit are input.
[0008]
According to the invention, there is provided an
optical signal processing device including: a first optica5 l
signal processing unit that generates a first digital signal
by causing a received optical signal which is received from
an outside and a local optical signal to interfere with each
other under a first condition; a second optical signal
10 processing unit that generates a second digital signal by
causing the received optical signal and the local optical
signal to interfere with each other under a second condition;
and a digital processing unit that processes the first
digital signal and the second digital signal and extracts
15 a signal included in the received optical signal, wherein
the first optical signal processing unit includes: a first
optical signal generation unit that generates a first
optical signal by causing the received optical signal and
the local optical signal to interfere with each other with
20 a same phase; a second optical signal generation unit that
generates a second optical signal by causing the received
optical signal and the local optical signal to interfere with
each other with a phase difference of π; a first
photoelectric conversion element that converts the first
25 optical signal into a first electrical signal; a second
photoelectric conversion element that converts the second
optical signal into a second electrical signal; a first
5
direct-current component correction unit that reduces a
difference between a direct-current component of the first
electrical signal and a direct-current component of the
second electrical signal; a differential first
trans-impedance amplifier into which the first electrica5 l
signal and the second electrical signal, which have been
corrected by the direct-current component correction unit,
are input; and a first AD conversion unit that converts an
output of the first trans-impedance amplifier into the first
10 digital signal, and the second optical signal processing
unit includes: a third optical signal generation unit that
generates a third optical signal by causing the received
optical signal and the local optical signal to interfere with
each other with a phase difference of π/2; a fourth optical
15 signal generation unit that generates a fourth optical
signal by causing the received optical signal and the local
optical signal to interfere with each other with a phase
difference of 3π/2; a third photoelectric conversion element
that converts the third optical signal into a third
20 electrical signal, a fourth photoelectric conversion
element that converts the fourth optical signal into a fourth
electrical signal; a second direct-current component
correction unit that reduces a difference between a
direct-current component of the third electrical signal and
25 a direct-current component of the fourth electrical signal;
a differential second trans-impedance amplifier into which
the third electrical signal and the fourth electrical signal,
6
which have been corrected by the direct-current component
correction unit, are input; and a second AD conversion unit
that converts an output of the second trans-impedance
amplifier into the second digital signal.
[00095 ]
According to the invention, there is provided an
optical signal processing method including: generating a
first optical signal by causing a received optical signal
which is received from an outside and a local optical signal
10 which is output from a light source on a receiving side to
interfere with each other with a first phase difference;
generating a second optical signal by causing the received
optical signal and the local optical signal to interfere with
each other with a second phase difference shifted by π from
15 the first phase difference; converting the first optical
signal into a first electrical signal; converting the second
optical signal into a second electrical signal; and reducing
a difference between a direct-current component of the first
electrical signal and a direct-current component of the
20 second electrical signal, and then inputting the first
electrical signal and the second electrical signal to a
differential trans-impedance circuit.
[0010]
According to the invention, there is provided an
25 optical signal processing device including: a first optical
signal generation unit that generates a first optical signal
by causing a received optical signal which is received from
7
an outside and a local optical signal to interfere with each
other with a first phase difference; a second optical signal
generation unit that generates a second optical signal by
causing the received optical signal and the local optical
signal to interfere with each other with a second phas5 e
difference shifted by π from the first phase difference; a
first photoelectric conversion element that converts the
first optical signal into a first electrical signal; a second
photoelectric conversion element that converts the second
10 optical signal into a second electrical signal; a
direct-current component correction unit that reduces a
direct-current component of the first electrical signal and
a direct-current component of the second electrical signal;
and a differential trans-impedance circuit into which the
15 first electrical signal and the second electrical signal,
which have been corrected by the direct-current component
correction unit, are input.
[0011]
According to the invention, there is provided a
20 trans-impedance amplifier including: a direct-current
component correction unit that reduces a difference between
a magnitude of a direct-current component of a first
electrical signal and a magnitude of a direct-current
component of a second electrical signal; and a differential
25 trans-impedance circuit into which the first electrical
signal and the second electrical signal, which have been
corrected by the direct-current component correction unit,
8
are input.
[0012]
According to the invention, there is provided an
optical signal processing device including: a first optical
signal processing unit that generates a first digital signa5 l
by causing a received optical signal which is received from
an outside and a local optical signal to interfere with each
other under a first condition; a second optical signal
processing unit that generates a second digital signal by
10 causing the received optical signal and the local optical
signal to interfere with each other under a second condition;
and a digital processing unit that processes the first
digital signal and the second digital signal and extracts
a signal included in the received optical signal, wherein
15 the first optical signal processing unit includes: a first
optical signal generation unit that generates a first
optical signal by causing the received optical signal and
the local optical signal to interfere with each other with
a same phase; a second optical signal generation unit that
20 generates a second optical signal by causing the received
optical signal and the local optical signal to interfere with
each other with a phase difference of π; a first
photoelectric conversion element that converts the first
optical signal into a first electrical signal; a second
25 photoelectric conversion element that converts the second
optical signal into a second electrical signal; a first
direct-current component correction unit that reduces a
9
direct-current component of the first electrical signal and
reduces a direct-current component of the second electrical
signal; a differential first trans-impedance amplifier to
which the first electrical signal and the second electrical
signal after corrected by the direct-current componen5 t
correction unit are input; and a first AD conversion unit
that converts an output of the first trans-impedance
amplifier into the first digital signal; and the second
optical signal processing unit includes: a third optical
10 signal generation unit that generates a third optical signal
by causing the received optical signal and the local optical
signal to interfere with each other with a phase difference
of π/2; a fourth optical signal generation unit that
generates a fourth optical signal by causing the received
15 optical signal and the local optical signal to interfere with
each other with a phase difference of 3π/2; a third
photoelectric conversion element that converts the third
optical signal into a third electrical signal, a fourth
photoelectric conversion element that converts the fourth
20 optical signal into a fourth electrical signal; a second
direct-current component correction unit that reduces a
direct-current component of the third electrical signal, and
reduces a direct-current component of the fourth electrical
signal; a differential second trans-impedance amplifier to
25 which the third electrical signal and the fourth electrical
signal after corrected by the direct-current component
correction unit are input; and a second AD conversion unit
10
that converts an output of the second trans-impedance
amplifier into the second digital signal.
[0013]
According to the invention, it is possible to suppress
an increase in the CMRR of an optical signal processin5 g
device. In addition, according to the invention, it is
possible to increase a dynamic range of an optical signal
processing device.
10 BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
The above-mentioned objects, other objects, features
and advantages will be made clearer with the preferred
embodiments described below and the following accompanying
15 drawings.
[0015]
Fig. 1 is a block diagram illustrating a configuration
of an optical signal processing device according to a first
embodiment.
20 Fig. 2 is a diagram illustrating the details of a
configuration of a direct-current component correction
unit.
Fig. 3 is a diagram illustrating a first effect of the
direct-current component correction unit.
25 Fig. 4 is a diagram illustrating the first effect of
the direct-current component correction unit.
Fig. 5 is a diagram illustrating a second effect of the
11
direct-current component correction unit.
Fig. 6 is a diagram illustrating the second effect of
the direct-current component correction unit.
Fig. 7 is a diagram illustrating a relationship between
optical powers of a received optical signal and local ligh5 t
which is defined by the Standard Committee of OIF.
Fig. 8 is a diagram illustrating a configuration of a
direct-current component correction unit according to a
second embodiment.
10 Fig. 9 is a diagram illustrating a configuration of a
direct-current component correction unit according to a
third embodiment.
Fig. 10 is a diagram illustrating a configuration of
a direct-current component correction unit according to a
15 fourth embodiment.
Fig. 11 is a diagram illustrating of a configuration
of a direct-current component correction unit according to
a fifth embodiment.
Fig. 12 is a diagram illustrating a configuration of
20 a direct-current component correction unit according to a
sixth embodiment.
Fig. 13 is a diagram illustrating a configuration of
a modified example of the direct-current component
correction unit according to the first embodiment.
25 Fig. 14 is a diagram illustrating a configuration of
a modified example of the direct-current component
correction unit according to the second embodiment.
12
Fig. 15 is a diagram illustrating a configuration of
an optical signal processing device according to a seventh
embodiment.
Fig. 16 is a diagram illustrating an operation of the
first embodiment5 .
Fig. 17 is a diagram illustrating an operation of the
first embodiment.
Fig. 18 is a diagram illustrating an effect of the first
embodiment.
10
DESCRIPTION OF EMBODIMENTS
[0016]
Hereinafter, embodiments of the present invention will
be described with reference to the accompanying drawings.
15 In all the drawings, the same elements are referenced by the
same reference numerals and descriptions thereof will not
be repeated.
[0017]
(First Embodiment)
20 Fig. 1 is a block diagram illustrating a configuration
of an optical signal processing device 10 according to a
first embodiment. The optical signal processing device 10
is a device that receives an optical signal, for example,
using a digital coherent scheme. The optical signal
25 processing device 10 includes an optical hybrid 100, four
photoelectric conversion elements 150, differential
trans-impedance amplifiers 200 and 202, two AD conversion
13
units 300, and a digital signal processing unit 400.
[0018]
An optical signal received from the outside is split
into an X polarized wave and a Y polarized wave by a
polarization beam splitter before the signal is input to th5 e
optical signal processing device 10. Either the X polarized
wave or the Y polarized wave is input into the optical signal
processing device 10. The optical signal processing device
10 splits the X polarized wave or the Y polarized wave of
10 the received optical signal which is input thereto, into an
optical signal for an in-phase baseband signal (I) and an
optical signal for a quadrature baseband signal (Q). Both
the optical signal for an in-phase baseband signal (I) and
the optical signal for a quadrature baseband signal (Q) are
15 polarized, and their polarization directions are orthogonal.
The differential trans-impedance amplifiers 200 and 202, and
the AD conversion unit 300 perform a coherent detection (for
example, homodyne detection or heterodyne detection) on the
optical signal for an in-phase baseband signal (I) and the
20 optical signal for a quadrature baseband signal (Q), and
convert these optical signals into an in-phase baseband
signal (I) and a quadrature baseband signal (Q). The
digital signal processing unit 400 reproduces a transmitted
multilevel modulation optical signal from the in-phase
25 baseband signal and the quadrature baseband signal, and
demodulates this multilevel modulation optical signal.
[0019]
14
The optical hybrid 100 generates a first optical signal
by causing local light to interfere with the X polarized wave
(or Y polarized wave) of the optical signal received from
the outside with a first phase difference. In addition, the
optical hybrid 100 generates a second optical signal 5 by
causing the local light to interfere with the X polarized
wave (or Y polarized wave) of the received optical signal
with a second phase difference shifted by π from the first
phase difference.
10 [0020]
Specifically, the optical hybrid 100 includes optical
mixers 112 (first optical signal generation unit), 114
(second optical signal generation unit), 122 (third optical
signal generation unit), and 124 (fourth optical signal
15 generation unit), and optical phase shifters 116, 126, and
128. The X polarized wave (or Y polarized wave) of the
received optical signal is input into each of the optical
mixers 112, 114, 122, and 124. The local light is input into
the optical mixer 112 without passing through any of the
20 phase shifters. The local light is input into the optical
mixer 114 through the optical phase shifter 116. The
optical phase shifter 116 shifts the phase of the local light
by π. The local light is input into the optical mixer 122
through the optical phase shifter 126. The optical phase
25 shifter 126 shifts the phase of the local light by π/2 in
the same direction as that of the optical phase shifter 116.
The local light is input into the optical mixer 124 through
15
the optical phase shifter 126 and the optical phase shifter
126. The optical phase shifter 128 shifts the phase of the
local light by π in the same direction as that of the optical
phase shifter 126.
[00215 ]
The optical mixer 112 generates a first optical signal
by causing the X polarized wave (or Y polarized wave) of the
received optical signal and the local light to interfere with
each other in the same phase, and the optical mixer 114
10 generates a second optical signal by causing the X polarized
wave (or Y polarized wave) of the received optical signal
and the local light to interfere with each other with a phase
difference of π. The optical mixer 122 generates a third
optical signal by causing the X polarized wave (or Y
15 polarized wave) of the received optical signal and the local
light to interfere with each other with a phase difference
of π/2, and the optical mixer 124 generates a fourth optical
signal by causing the X polarized wave (or Y polarized wave)
of the received optical signal and the local light to
20 interfere with each other with a phase difference of 3π/2.
The first optical signal and the second optical signal form
a set of signals, and the third optical signal and the fourth
optical signal also form a set of signals.
[0022]
25 Four photoelectric conversion elements 150
photoelectrically convert the first optical signal, the
second optical signal, the third optical signal, and the
16
fourth optical signal, and generate a first electrical
signal, a second electrical signal, a third electrical
signal, and a fourth electrical signal, respectively. The
photoelectric conversion element 150 is, for example, a
photodiode. The first electrical signal and the secon5 d
electrical signal are input into the differential
trans-impedance amplifier 200, and the third electrical
signal and the fourth electrical signal are input into the
differential trans-impedance amplifier 202.
10 [0023]
The differential trans-impedance amplifier 200
includes a direct-current component correction unit 210, a
trans-impedance circuit 240, and a variable gain amplifier
250. The direct-current component correction unit 210
15 reduces the difference between the magnitude of a
direct-current component of the first electrical signal and
the magnitude of a direct-current component of the second
electrical signal. The details of the configuration of the
direct-current component correction unit 210 will be
20 described later. The first electrical signal and the second
electrical signal, the direct-current components of which
have been corrected by the direct-current component
correction unit 210, are input into the trans-impedance
circuit 240. The variable gain amplifier 250 amplifies the
25 magnitude of an output of the trans-impedance circuit 240,
and outputs the amplified output to the AD conversion unit
300.
17
[0024]
Meanwhile, the differential trans-impedance amplifier
202 also includes the direct-current component correction
unit 210, the trans-impedance circuit 240, and the variable
gain amplifier 250. The differential trans-impedanc5 e
amplifier 202 has the same function as that of the
differential trans-impedance amplifier 200, except that the
third electrical signal is input thereto instead of the first
electrical signal, and that the fourth electrical signal is
10 input thereto instead of the second electrical signal.
[0025]
The AD conversion unit 300 converts two analog signals
output from the differential trans-impedance amplifier 200
into a digital signal. This digital signal is an in-phase
15 baseband signal.
[0026]
The AD conversion unit 302 converts two analog signals
output from the differential trans-impedance amplifier 202
into a digital signal. This digital signal is a quadrature
20 baseband signal.
[0027]
Meanwhile, when photoelectric currents of two
photoelectric conversion elements 150 are set to I1 and I2,
the common mode rejection ratio (CMRR) is expressed by the
25 following Expression (1).
[0028]
18
··· (1)
[0029]
Herein, the reason that the CMRR influences the
accuracy of signal processing of a digital coherent schem5 e
will be described by taking an example of a quadrature phase
shift keying (QPSK) optical signal. As mentioned above, in
the optical hybrid 100, the received optical signal input
into a first input unit 102 is converted into the first
10 optical signal, the second optical signal, the third optical
signal, and the fourth optical signal. The received optical
signal input into the first input unit 102 is expressed by
the following Expression (2), and the local light is
expressed by the following Expression (3).
15 [0030]
··· (2)
··· (3)
20 [0031]
Meanwhile, ω1 is the frequency of the received optical
signal, and ω is the frequency of the local light. Here,
φ is a phase. In a case of QPSK, the relation of φ=0 is
established in the generation of the first optical signal,
25 the relation of φ=π is established in the generation of the
19
second optical signal, the relation of φ=π/2 is established
in the generation of the third optical signal, and the
relation of φ=3π/2 is established in the generation of the
fourth optical signal.
[00325 ]
Here, since the relation of ω1=ω is established, the
first optical signal is expressed by the following
Expression (4), the second optical signal is expressed by
the following Expression (5), the third optical signal is
10 expressed by the following Expression (6), and the fourth
optical signal is expressed by the following Expression (7).
[0033]
··· (4)
15
··· (5)
··· (6)
20 ··· (7)
[0034]
Herein, a, b, c, and d are coefficients caused by the
quantum efficiency of the photoelectric conversion element
20
150 or the loss in the optical hybrid 100. In each of
Expressions (4) to (7), a first term and a second term are
DC components (offset components), and a third term is phase
information of a signal.
[00355 ]
The outputs of the trans-impedance circuit 240 of the
differential trans-impedance amplifier 200 are expressed by
the following Expressions (8) and (9). In addition, the
outputs of the trans-impedance circuit 240 of the
10 differential trans-impedance amplifier 202 are expressed by
the following Expressions (10) and (11).
[0036]
15
[0037]
The differential signals of Expressions (8) and (9) are
input into the variable gain amplifier 250 of the
20 differential trans-impedance amplifier 200. In addition,
the differential signals of Expressions (10) and (11) are
input into the variable gain amplifier 250 of the
differential trans-impedance amplifier 200.
[0038]
Herein, since the light intensity B2 25 of the local light
21
is more than 10 times the intensity A2 of the received optical
signal, B2 becomes dominant in Expressions (8) to (11). For
this reason, when a difference occurs between the
coefficients a and b of Expressions (8) and (9), the DC level
of the output signal of the trans-impedance circuit 240 o5 f
the differential trans-impedance amplifier 200 changes
considerably. In addition, when a difference occurs
between the coefficients c and d of Expressions (10) and (11),
the DC level of the output signal of the trans-impedance
10 circuit 240 of the differential trans-impedance amplifier
202 changes considerably. For this reason, when the CMRR
is insufficient, the accuracy of waveform distortion
equalization by the variable gain amplifier 250 or the
digital signal processing unit 400 is deteriorated.
15 [0039]
On the other hand, in the embodiment, the
direct-current component correction unit 210 reduces the
difference between the magnitude of the direct-current
component of the first electrical signal and the magnitude
20 of the direct-current component of the second electrical
signal (or difference between the magnitude of the
direct-current component of the third electrical signal and
the magnitude of the direct-current component of the fourth
electrical signal). Therefore, an increase in the CMRR is
25 suppressed, and thus the deterioration in the accuracy of
signal processing by the optical signal processing device
10 can be suppressed.
22
[0040]
Fig. 2 is a diagram illustrating the details of the
configuration of the direct-current component correction
unit 210 in the differential trans-impedance amplifier 200,
together with the optical hybrid 100 and the trans-impedanc5 e
circuit 240. Meanwhile in the drawing, the optical hybrid
100 is simplified as compared with Fig. 1.
[0041]
The direct-current component correction unit 210
10 includes a first transistor 222, a second transistor 224,
and a constant current source 230. In an example shown in
the drawing, the first transistor 222 and the second
transistor 224 are bipolar transistors, but may be field
effect transistors such as a MOS transistor. The first
15 transistor 222 and the second transistor 224 constitute a
differential circuit 220.
[0042]
The first transistor 222 is configured so that a
collector thereof (a drain in a case of a field effect
20 transistor) is connected to an interconnection that inputs
the first electrical signal into the direct-current
component correction unit 210, and an emitter thereof (a
source in a case of the field effect transistor) is connected
to the input side of the constant current source 230. The
25 second transistor 224 is configured so that a collector
thereof is connected to an interconnection that inputs the
second electrical signal into the direct-current component
23
correction unit 210, and an emitter thereof is connected to
the input side of the constant current source 230. The
output side of the constant current source 230 is grounded
through, for example, a resistor (not shown). Meanwhile,
the example of the above-mentioned connection shows a cas5 e
where the first transistor 222 and the second transistor 224
are NPN-type bipolar transistors. However, the first
transistor 222 and the second transistor 224 may be PNP-type
bipolar transistors. In this case, it is possible to obtain
10 the same effect as that in the above-mentioned example by
appropriately changing a design.
[0043]
Meanwhile, the direct-current component correction
unit 210 of the differential trans-impedance amplifier 202
15 also has the same configuration as that of the direct-current
component correction unit 210 of the differential
trans-impedance amplifier 200, except that the third
electrical signal is input into the collector of the first
transistor 222, and that the fourth electrical signal is
20 input into the collector of the second transistor 224.
[0044]
In addition, the trans-impedance circuit 240 includes
a differential amplifier, two emitter follower circuits, and
two feedback resistors.
25 [0045]
Next, the operation of the direct-current component
correction unit 210 will be described with reference to Fig.
24
16.
[0046]
When a direct-current component IPD1 of the first
electrical signal and a direct-current component IPD2 of the
second electrical signal are equal to each other, an inpu5 t
control voltage (base) V1 to the first transistor 222 and
an input control voltage (base) V2 to the second transistor
224 are set to be the same potential using a control unit
228. Thereby, a current Ic1 flowing through the first
10 transistor 222 and a current Ic2 flowing through the second
transistor 224 become equal to each other. In this case,
a direct-current component of the first electrical signal
(IPD1-Ic1) and a direct-current component of the second
electrical signal (IPD2-Ic2), which are input into the
15 trans-impedance circuit 240, become equal to each other.
[0047]
On the other hand, when the direct-current component
IPD1 of the first electrical signal and the direct-current
component IPD2 of the second electrical signal are different
20 from each other (for example, IPD1>IPD2), the input control
voltage (base) V1 to the first transistor 222 is set to be
higher than the input control voltage (base) V2 to the second
transistor 224, using the control unit 228. Thereby, the
current Ic1 flowing through the first transistor 222 becomes
25 larger than the current Ic2 flowing through the second
transistor 224 (IC1>IC2). Thereby, the difference between
the direct-current component of the first electrical signal
25
(IPD1-Ic1) and the direct-current component of the second
electrical signal (IPD2-Ic2), which are input into the
trans-impedance circuit 240, is reduced.
[0048]
Meanwhile, in the embodiment, the control unit 228 5 may
be included as shown in Fig. 13. The control unit 228
controls the input control voltage of the first transistor
222 and the input control voltage of the second transistor
224, on the basis of potentials of two output signals of the
10 trans-impedance circuit 240 (differential signals).
Specifically, the control unit 228 controls the control
voltage of the first transistor 222 on the basis of a
potential of a first output of the trans-impedance circuit
240 (P signal), and controls the control voltage of the
15 second transistor 224 on the basis of a potential of a second
output of the trans-impedance circuit 240 (N signal).
[0049]
However, the operation of the direct-current component
correction unit 210 according to the embodiment is not
20 limited only to the feedback control of the output of the
trans-impedance circuit 240. The first transistor 222 and
the second transistor 224 may operate through the control
unit 228 in accordance with a detection value based on
another detection method or an input from the outside.
25 However, even in this case, the details of the control by
the control unit 228 are as described with reference to Fig.
16.
26
[0050]
Fig. 3 shows an example of waveforms of output signals
after the demodulation by the trans-impedance circuit 240
in a case using a QPSK scheme in which the received optical
signal is 31.78911 Gb/s. Fig. 4 shows an example of outpu5 t
waveforms of the trans-impedance circuit 240 without the
direct-current component correction unit 210. Comparing
these examples, it is determined that a current difference
is corrected by providing the direct-current component
10 correction unit 210, and accordingly the output waveforms
of the trans-impedance circuit 240 become satisfactory.
[0051]
In addition, as shown in Fig. 5, when the direct-current
component of an electrical signal is large, the
15 trans-impedance circuit 240 for the electrical signal is
required to have a large input dynamic range. On the other
hand, according to the embodiment, as shown in Fig. 6, the
first transistor 222 and the second transistor 224 are
controlled, thereby the direct-current component of each the
20 first electrical signal and the second electrical signal can
be reduced (possibly each of them can be set to 0 depending
on the configuration of the direct-current component
correction unit 210). Thereby, it is possible to reduce the
input dynamic range that the trans-impedance circuit 240 is
25 required to have.
[0052]
This effect will be described in detail with reference
27
to Fig. 18. Fig. 18 shows a current signal of an output of
the photoelectric conversion element 150 in a case where the
received optical signal is -12 dBm, and the local optical
signal is 12 dBm. In this current signal, a modulation
signal is 0.4 mApp. On the other hand, the direct-curren5 t
component is 1.8 mA, and is larger than the modulation signal.
Even when such a current signal is input into the
trans-impedance circuit 240, it is very difficult to
maintain to the linearity of the trans-impedance circuit 240.
10 As a result, distortion occurs in a signal to be demodulated.
[0053]
On the other hand, according to the embodiment, a
direct-current component in Fig. 18 is caused to be reduced,
and thus the ratio of the modulation signal to the
15 direct-current component can be increased. Therefore, it
is possible to maintain the linearity of the trans-impedance
circuit 240. In order to express these effects, Fig. 17
shows a relationship between the currents Ic1 and Ic2 flowing
through the first transistor 222 and the second transistor
20 224. The difference between Ic1 and Ic2 compensates for the
difference between the DC currents of IPD1 and IPD2, and an
excessive DC current of the PD is suppressed by the same
current value of Ic1 and Ic2.
[0054]
25 Fig. 7 shows a relationship between optical powers of
the received optical signal and the local light which is
defined by the Standard Committee of Optical Internetworking
28
Forum (OIF). It shows that the optical power of the local
light is more than 10 times that of the received optical
signal. Since a coherent receiver needs to have high
linearity, a differential trans-impedance amplifier is
required to have a wide input dynamic range. In th5 e
embodiment, as mentioned above, it is possible to reduce a
dynamic range that the trans-impedance circuit 240 is
required to have.
[0055]
10 (Second Embodiment)
Fig. 8 is a diagram illustrating a configuration of the
direct-current component correction unit 210 used in the
optical signal processing device 10 according to a second
embodiment. The optical signal processing device 10
15 according to the embodiment has the same configuration as
that of the direct-current component correction unit 210
according to the first embodiment shown in Fig. 2, except
that the constant current source 230 of the direct-current
component correction unit 210 is a current mirror circuit.
20 [0056]
This current mirror circuit includes transistors 232
and 234. The transistors 232 and 234 are, for example,
bipolar transistors, but may be field effect transistors.
A base of the transistor 234 is connected to a collector.
25 A collector of the transistor 232 is connected to the
emitters of the first transistor 222 and the second
transistor 224, and the collector of the transistor 234 is
29
connected to the outside.
[0057]
In the embodiment, it is also possible to obtain the
same effect as that in the first embodiment. In addition,
in the embodiment, an input to the collector of th5 e
transistor 234 of the current mirror circuit is controlled,
thereby allowing the direct-current components of the first
electrical signal and the second electrical signal to be
reduced. Therefore, it is possible to particularly reduce
10 the size of the dynamic range that the trans-impedance
circuit 240 is required to have.
[0058]
Meanwhile, in the embodiment, the control unit 228 may
also be included as shown in the drawing. Note that, in the
15 direct-current component correction unit 210, similarly to
the direct-current component correction unit 210 according
to the first embodiment, the first transistor 222 and the
second transistor 224 may operate not only with the feedback
control from the output of the trans-impedance circuit 240,
20 but also with another detection method or the control unit
228 from the outside. The details of the control are as
described with reference to Figs. 16 and 17.
[0059]
(Third Embodiment)
25 Fig. 9 is a diagram illustrating a configuration of the
control unit 228 of the optical signal processing device 10
according to a third embodiment, together with another
30
configuration of the direct-current component correction
unit 210 and the trans-impedance circuit 240. The optical
signal processing device 10 according to the embodiment has
the same configuration as that of the optical signal
processing device 10 according to the second embodiment5 ,
except for the configuration of the control unit 228.
[0060]
The control unit 228 according to the embodiment
includes an integration unit 270 and a level conversion unit
10 280. The integration unit 270 includes two integration
circuits. These two integration circuits integrate each of
two output signals of the trans-impedance circuit 240, and
detect a potential of each of the output signals. The level
conversion unit 280 converts each of the output levels of
15 the two integration circuits. Two outputs of the level
conversion unit 280 are input into the collector of the first
transistor 222 and the collector of the second transistor
224, respectively.
[0061]
20 An effect of the optical signal processing device 10
according to the embodiment will be described below. When
the direct-current component IPD1 of the first electrical
signal and the direct-current component IPD2 of the second
electrical signal are equal to each other (IPD1=IPD2),
25 correction signals having the same level are output to
outputs OUTP and OUTN of the trans-impedance circuit 240.
For this reason, the input control voltage (base) to the
31
first transistor 222 and the input control voltage (base)
to the second transistor 224 are set to the same potential
through the level conversion circuit 280. Thereby, the
current Ic1 flowing through the first transistor 222 and the
current Ic2 flowing through the second transistor 224 becom5 e
equal to each other. In this case, the direct-current
component of the first electrical signal (IPD1-Ic1) and the
direct-current component of the second electrical signal
(IPD2-Ic2) which are input into the trans-impedance circuit
10 240 become equal to each other.
[0062]
In addition, a voltage terminal VCM of the current
mirror circuit 230 is adjusted, thereby allowing the
direct-current components of the first electrical signal and
15 the second electrical signal, which are input into the
trans-impedance circuit 11, to be reduced to 0. In this
manner, it is possible to obtain the sufficient input dynamic
range of the trans-impedance circuit 240.
[0063]
20 On the other hand, when a difference occurs between the
direct-current component IPD1 of the first electrical signal
and the direct-current component IPD2 of the second
electrical signal (for example, IPD1>IPD2), normally,
correction signals having different levels are output to the
25 outputs OUTP and OUTN of the trans-impedance circuit 240.
These are detected by the integration circuit 20.
[0064]
32
The difference between the levels detected by the
integration circuit 270 is converted to have a proper voltage
range through the level conversion circuit 280. Each of the
input control voltage (base) to the first transistor 222 and
the input control voltage (base) of the second transisto5 r
224 are set to proper values according to the level
difference after the conversion. The relation between the
current Ic1 flowing through the first transistor 222 and the
current Ic2 flowing through the second transistor 224 becomes
10 Ic1>Ic2. The above-mentioned operation is repeated until
the levels of the outputs OUTP and OUTN of the
trans-impedance circuit 240 become the same as each other.
As a result, signals of which the direct-current component
of the first electrical signal (IPD1-Ic1) and the
15 direct-current component of the second electrical signal
(IPD2-Ic2) are the same as each other are automatically input
to the trans-impedance circuit 240.
[0065]
In the embodiment, it is also possible to obtain the
20 same effect as that in the second embodiment.
[0066]
(Fourth Embodiment)
Fig. 10 is a diagram illustrating a configuration of
the differential trans-impedance amplifier 200 of the
25 optical signal processing device 10 according to a fourth
embodiment. The differential trans-impedance amplifier
200 according to the embodiment has the same configuration
33
as that of the optical signal processing device 10 according
to the third embodiment, except that the constant current
source 230 of the direct-current component correction unit
210 is a third transistor 290. Meanwhile, in the embodiment
and all the other embodiments, it is also possible to replac5 e
a bipolar transistor with a field effect transistor.
[0067]
Meanwhile, the differential trans-impedance amplifier
202 shown in Fig. 1 also has the same configuration as that
10 of the differential trans-impedance amplifier 200.
In the embodiment, it is possible to obtain the same
effect as that in the third embodiment.
[0068]
(Fifth Embodiment)
15 Fig. 11 is a diagram illustrating a configuration of
the direct-current component correction unit 210 of the
optical signal processing device 10 according to a fifth
embodiment, together with the configuration of the
trans-impedance circuit 240. The optical signal processing
20 device 10 according to the embodiment has the same
configuration as that of the optical signal processing
device 10 according to the first embodiment, except that the
differential trans-impedance amplifier 200 does not include
the control unit 228, and that the direct-current component
25 correction unit 210 has a different configuration.
[0069]
The direct-current component correction unit 210
34
includes the first transistor 222, the second transistor 224,
and a fourth transistor 226. In the embodiment, the same
signal is input into a base of the first transistor 222 and
a base of the second transistor 224 from the control unit
228. In addition, a base and a collector of the fourt5 h
transistor 226 are connected to the base of the second
transistor 224. Emitters of the first transistor 222, the
second transistor 224, and the fourth transistor 226 are all
grounded through, for example, a resistor (not shown).
10 [0070]
According to the embodiment, inputs to the base of the
first transistor 222, the base of the second transistor 224,
and the collector of the fourth transistor 226 are controlled,
thereby allowing each of the direct-current components of
15 the first electrical signal and the second electrical signal
which are input into the trans-impedance circuit 240 to be
reduced. Thereby, it is possible to reduce the dynamic
range that the trans-impedance circuit 240 is required to
have.
20 (Sixth Embodiment)
Fig. 12 is a diagram illustrating a configuration of
the direct-current component correction unit 210 of the
optical signal processing device 10 according to a sixth
embodiment, together with the configuration of the
25 trans-impedance circuit 240. The optical signal processing
device 10 according to the embodiment has the same
configuration as that of the optical signal processing
35
device 10 according to the fifth embodiment, except that the
differential trans-impedance amplifier 200 does not include
the control unit 228, and that the direct-current component
correction unit 210 does not include the fourth transistor
5 226.
In the embodiment, it is also possible to obtain the
same effect as that in the fifth embodiment.
[0071]
(Seventh Embodiment)
10 Fig. 15 is a diagram illustrating a configuration of
an optical signal processing device according to a seventh
embodiment. The optical signal processing unit according
to the embodiment is an optical signal processing unit that
receives an optical signal using a digital coherent scheme.
15 This optical signal processing unit includes an optical
signal processing device 12, an electrical signal processing
device 20, and a local light source 500.
[0072]
The optical signal processing device 12 includes two
20 signal processing units 14. Both the signal processing
units 14 include the optical hybrid 100, four photoelectric
conversion elements 150, and differential trans-impedance
amplifiers 200 and 202. The optical hybrid 100, the four
photoelectric conversion elements 150, and the differential
25 trans-impedance amplifiers 200 and 202 of the signal
processing unit 14 have the same configurations as those of
the optical hybrid 100, the four photoelectric conversion
36
elements 150, and the differential trans-impedance
amplifiers 200 and 202 which are shown in the first to sixth
embodiments.
[0073]
A received signal light which is input into the 5 first
input unit 102 of the optical signal processing device 12
is split into an X polarized wave and a Y polarized wave by
a polarization beam splitter 600. Each of the X polarized
wave and the Y polarized wave are input into the different
10 signal processing units 14.
[0074]
The local light source 500 is connected to a second
input unit 104 of the optical signal processing device 12.
The local light source 500 inputs local light to the second
15 input unit 104. The local light input into the second input
unit 140 is split into two beams of light by a beam splitter
602. Each of the two beams of light are input into the
different signal processing units 14.
[0075]
20 The electrical signal processing device 20 includes two
AD conversion groups 304 and the digital signal processing
unit 400. Both the AD conversion groups 304 include the AD
conversion units 300 and 302. A signal is input into the
first AD conversion group 304 from the first signal
25 processing unit 14 included in the optical signal processing
device 12, and a signal is input into the second AD conversion
group 304 from the second signal processing unit 14 included
37
in the optical signal processing device 12. The digital
signal processing unit 400 processes outputs from the two
AD conversion groups 304, and generates a demodulated
signal.
[00765 ]
In the embodiment, it is also possible to obtain the
same effects as that in first to sixth embodiments.
[0077]
As described above, although the embodiments of the
10 invention have been set forth with reference to the drawings,
they are merely illustrative of the invention, and various
configurations other than stated above can be adopted.
[0078]
This application claims priority to Japanese Patent
15 Application No. 2011-208714 filed on September 26, 2011, the
content of which is incorporated herein by reference in its
entirety.
38
CLAIMS
1. An optical signal processing device comprising:
a first optical signal generation unit that generates
a first optical signal by causing a received optical signal
which is received from an outside and a local optical signal
to interfere with each other with a first phase difference5 ;
a second optical signal generation unit that generates
a second optical signal by causing the received optical
signal and the local optical signal to interfere with each
other with a second phase difference shifted by π from the
10 first phase difference;
a first photoelectric conversion element that converts
the first optical signal into a first electrical signal;
a second photoelectric conversion element that
converts the second optical signal into a second electrical
15 signal;
a direct-current component correction unit that
reduces a difference between a magnitude of a direct-current
component of the first electrical signal and a magnitude of
a direct-current component of the second electrical signal;
20 and
a differential trans-impedance circuit into which the
first electrical signal and the second electrical signal,
which have been corrected by the direct-current component
correction unit, are input.
39
2. The optical signal processing device according to claim
1, wherein the direct-current component correction unit
includes:
a first transistor which is connected between the first
photoelectric conversion element and the trans-impedanc5 e
circuit;
a second transistor which is connected between the
second photoelectric conversion element and the
trans-impedance circuit; and
10 a constant current source which is connected between
the first photoelectric conversion element and the
trans-impedance circuit through the first transistor, and
connected between the second photoelectric conversion
element and the trans-impedance circuit through the second
15 transistor.
3. The optical signal processing device according to claim
2, further comprising a control unit that controls a control
voltage of the first transistor and a control voltage of the
second transistor,
5 wherein the control unit controls the control voltage
of the first transistor and the control voltage of the second
transistor, on the basis of two output signals of the
trans-impedance circuit.
4. The optical signal processing device according to claim
3, wherein the control unit includes:
40
two integration circuits that integrate the two output
signals of the trans-impedance circuit, respectively; and
a level conversion unit that converts an output leve5 l
of each of the two integration circuits, and
wherein two outputs of the level conversion unit are
input as a gate voltage of the first transistor and a gate
voltage of the second transistor.
5. The optical signal processing device according to any
one of claims 2 to 4, wherein the constant current source
includes a current mirror circuit.
6. The optical signal processing device according to claim
3 or 4, wherein the control unit controls the current mirror
circuit.
7. The optical signal processing device according to claim
1, wherein the direct-current component correction unit
includes:
a first transistor which is connected between the first
5 photoelectric conversion element and the trans-impedance
circuit;
a second transistor which is connected between the
second photoelectric conversion element and the
trans-impedance circuit;
10 a third transistor which is connected between the first
photoelectric conversion element and the trans-impedance
41
circuit through the first transistor, and connected between
the second photoelectric conversion element and the
trans-impedance circuit through the second transistor; and
a control unit that controls a control voltage of th15 e
first transistor and a control voltage of the second
transistor.
8. The optical signal processing device according to any
one of claims 1 to 7, further comprising an AD conversion
unit that converts two output signals of the trans-impedance
circuit into a digital signal.
9. The optical signal processing device according to any
one of claims 1 to 8, wherein the direct-current component
correction unit reduces each of the direct-current
components of the first electrical signal and the second
5 electrical signal.
10. The optical signal processing device according to any
one of claims 1 to 9, wherein the first phase difference is
0 or π/2.
11. An optical signal processing device comprising:
a first optical signal processing unit that generates
a first digital signal by causing a received optical signal
which is received from an outside and a local optical signal
5 to interfere with each other under a first condition;
42
a second optical signal processing unit that generates
a second digital signal by causing the received optical
signal and the local optical signal to interfere with each
other under a second condition; and
a digital processing unit that processes the 10 first
digital signal and the second digital signal and extracts
a signal included in the received optical signal,
wherein the first optical signal processing unit
includes:
15 a first optical signal generation unit that generates
a first optical signal by causing the received optical signal
and the local optical signal to interfere with each other
with a same phase;
a second optical signal generation unit that generates
20 a second optical signal by causing the received optical
signal and the local optical signal to interfere with each
other with a phase difference of π;
a first photoelectric conversion element that converts
the first optical signal into a first electrical signal;
25 a second photoelectric conversion element that
converts the second optical signal into a second electrical
signal;
a first direct-current component correction unit that
reduces a difference between a direct-current component of
30 the first electrical signal and a direct-current component
of the second electrical signal;
a differential first trans-impedance amplifier into
43
which the first electrical signal and the second electrical
signal, which have been corrected by the direct-current
component correction unit, are input; 35 and
a first AD conversion unit that converts an output of
the first trans-impedance amplifier into the first digital
signal, and
the second optical signal processing unit includes:
40 a third optical signal generation unit that generates
a third optical signal by causing the received optical signal
and the local optical signal to interfere with each other
with a phase difference of π/2;
a fourth optical signal generation unit that generates
45 a fourth optical signal by causing the received optical
signal and the local optical signal to interfere with each
other with a phase difference of 3π/2;
a third photoelectric conversion element that converts
the third optical signal into a third electrical signal;
50 a fourth photoelectric conversion element that
converts the fourth optical signal into a fourth electrical
signal;
a second direct-current component correction unit that
reduces a difference between a direct-current component of
55 the third electrical signal and a direct-current component
of the fourth electrical signal;
a differential second trans-impedance amplifier into
which the third electrical signal and the fourth electrical
signal, which have been corrected by the direct-current
44
component correction unit, are input; 60 and
a second AD conversion unit that converts an output of
the second trans-impedance amplifier into the second digital
signal.
12. The optical signal processing device according to claim
11, wherein both the first direct-current component
correction unit and the second direct-current component
correction unit include:
5 a first transistor which is connected between the first
photoelectric conversion element and the trans-impedance
circuit;
a second transistor which is connected between the
second photoelectric conversion element and the
10 trans-impedance circuit; and
a constant current source which is connected between
the first photoelectric conversion element and the
trans-impedance circuit through the first transistor, and
connected between the second photoelectric conversion
15 element and the trans-impedance circuit through the second
transistor.
13. The optical signal processing device according to claim
12, further comprising a control unit that controls a control
voltage of the first transistor and a control voltage of the
second transistor,
5 wherein the control unit controls the control voltage
45
of the first transistor and the control voltage of the second
transistor, on the basis of two output signals of the
trans-impedance circuit.
14. The optical signal processing device according to claim
13, wherein the control unit includes:
two integration circuits that integrate the two output
signals of the trans-impedance circuit, respectively; and
a level conversion unit that converts an output leve5 l
of each of the two integration circuits, and
wherein two outputs of the level conversion unit are
input as a gate voltage of the first transistor and a gate
voltage of the second transistor.
15. The optical signal processing device according to any
one of claims 12 to 14, wherein the constant current source
includes a current mirror circuit.
16. The optical signal processing device according to claim
15, further comprising a second control unit that controls
the current mirror circuit.
17. The optical signal processing device according to any
one of claims 11 to 16,
wherein the first direct-current component correction
unit reduces each of the direct-current components of the
5 first electrical signal and the second electrical signal,
46
and
wherein the second direct-current component correction
unit reduces each of the direct-current components of the
third electrical signal and the fourth electrical signal.
18. A trans-impedance amplifier comprising:
a direct-current component correction unit that
reduces a difference between a magnitude of a direct-current
component of a first electrical signal and a magnitude of
a direct-current component of a second electrical signal5 ;
and
a differential trans-impedance circuit into which the
first electrical signal and the second electrical signal,
which have been corrected by the direct-current component
10 correction unit, are input.
19. An optical signal processing method comprising:
generating a first optical signal by causing a received
optical signal which is received from an outside and a local
optical signal which is output from a light source on a
5 receiving side to interfere with each other with a first phase
difference;
generating a second optical signal by causing the
received optical signal and the local optical signal to
interfere with each other with a second phase difference
10 shifted by π from the first phase difference;
converting the first optical signal into a first
47
electrical signal;
converting the second optical signal into a second
electrical signal; and
reducing a difference between a direct-curren15 t
component of the first electrical signal and a direct-current
component of the second electrical signal, and then inputting
the first electrical signal and the second electrical signal
to a differential trans-impedance circuit.
20. An optical signal processing device comprising:
a first optical signal generation unit that generates
a first optical signal by causing a received optical signal
which is received from an outside and a local optical signal
5 to interfere with each other with a first phase difference;
a second optical signal generation unit that generates
a second optical signal by causing the received optical
signal and the local optical signal to interfere with each
other with a second phase difference shifted by π from the
10 first phase difference;
a first photoelectric conversion element that converts
the first optical signal into a first electrical signal;
a second photoelectric conversion element that
converts the second optical signal into a second electrical
15 signal;
a direct-current component correction unit that
reduces a direct-current component of the first electrical
signal and a direct-current component of the second
48
electrical signal; and
a differential trans-impedance circuit into which th20 e
first electrical signal and the second electrical signal,
which have been corrected by the direct-current component
correction unit, are input.
21. An optical signal processing device comprising:
a first optical signal processing unit that generates
a first digital signal by causing a received optical signal
which is received from an outside and a local optical signal
5 to interfere with each other under a first condition;
a second optical signal processing unit that generates
a second digital signal by causing the received optical
signal and the local optical signal to interfere with each
other under a second condition; and
10 a digital processing unit that processes the first
digital signal and the second digital signal and extracts
a signal included in the received optical signal,
wherein the first optical signal processing unit
includes:
15 a first optical signal generation unit that generates
a first optical signal by causing the received optical signal
and the local optical signal to interfere with each other
with a same phase;
a second optical signal generation unit that generates
20 a second optical signal by causing the received optical
signal and the local optical signal to interfere with each
49
other with a phase difference of π;
a first photoelectric conversion element that converts
the first optical signal into a first electrical signal;
a second photoelectric conversion element tha25 t
converts the second optical signal into a second electrical
signal;
a first direct-current component correction unit that
reduces a direct-current component of the first electrical
30 signal, and reduces a direct-current component of the second
electrical signal;
a differential first trans-impedance amplifier into
which the first electrical signal and the second electrical
signal, which have been after corrected by the direct-current
35 component correction unit, are input, and
a first AD conversion unit that converts an output of
the first trans-impedance amplifier into the first digital
signal, and
the second optical signal processing unit includes:
40 a third optical signal generation unit that generates
a third optical signal by causing the received optical signal
and the local optical signal to interfere with each other
with a phase difference of π/2;
a fourth optical signal generation unit that generates
45 a fourth optical signal by causing the received optical
signal and the local optical signal to interfere with each
other with a phase difference of 3π/2;
a third photoelectric conversion element that converts
50
the third optical signal into a third electrical signal;
a fourth photoelectric conversion element tha50 t
converts the fourth optical signal into a fourth electrical
signal;
a second direct-current component correction unit that
reduces a direct-current component of the third electrical
55 signal, and reduces a direct-current component of the fourth
electrical signal;
a differential second trans-impedance amplifier into
which the third electrical signal and the fourth electrical
signal, which have been corrected by the direct-current
60 component correction unit, are input; and
a second AD conversion unit that converts an output of
the second trans-impedance amplifier into the second digital
signal.