U.S. patent application number 14/418386 was filed with the patent office on 2015-09-17 for light modulation circuit.
The applicant listed for this patent is NIPPON TELEGRAPH AND TELEPHONE CORPORATION. Invention is credited to Takashi Goh, Kiyofumi Kikuchi, Takashi Saida, Hiroshi Takahashi, Hiroshi Yamazaki.
Application Number | 20150261059 14/418386 |
Document ID | / |
Family ID | 50387555 |
Filed Date | 2015-09-17 |
United States Patent
Application |
20150261059 |
Kind Code |
A1 |
Yamazaki; Hiroshi ; et
al. |
September 17, 2015 |
LIGHT MODULATION CIRCUIT
Abstract
An optical modulation circuit includes: a first Mach-Zehnder
modulating portion including a first output port and a second
output port, wherein the first Mach-Zehnder modulating portion is
push-pull driven by a main signal; a second Mach-Zehnder modulating
portion connected to the first output port of the first
Mach-Zehnder modulating portion, wherein the second Mach-Zehnder
modulating portion is push-pull driven by a correction signal; and
an asymmetric light combining portion combining an optical signal
outputted from an output port of the second Mach-Zehnder modulating
portion with an optical signal outputted from the second output
port of the first Mach-Zehnder modulating portion in a light
intensity coupling ratio of r to 1-r, wherein an optical path
length from the first output port to the asymmetric light combining
portion is substantially equal to an optical path length from the
second output port to the asymmetric light combining portion.
Inventors: |
Yamazaki; Hiroshi;
(Atsugi-shi, JP) ; Takahashi; Hiroshi;
(Atsugi-shi, JP) ; Goh; Takashi; (Atsugi-shi,
JP) ; Saida; Takashi; (Atsugi-shi, JP) ;
Kikuchi; Kiyofumi; (Atsugi-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON TELEGRAPH AND TELEPHONE CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
50387555 |
Appl. No.: |
14/418386 |
Filed: |
September 26, 2013 |
PCT Filed: |
September 26, 2013 |
PCT NO: |
PCT/JP2013/005744 |
371 Date: |
January 29, 2015 |
Current U.S.
Class: |
385/3 |
Current CPC
Class: |
G02F 1/035 20130101;
G02F 2201/16 20130101; G02F 2001/212 20130101; G02F 1/2257
20130101; G02F 1/2255 20130101; H04B 10/588 20130101; G02F 1/0123
20130101; G02F 2201/126 20130101; G02F 1/225 20130101; G02F 2203/19
20130101; H04B 10/5051 20130101 |
International
Class: |
G02F 1/225 20060101
G02F001/225 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2012 |
JP |
2012-217233 |
Claims
1. An optical modulation circuit, comprising: a first Mach-Zehnder
modulating portion including a first output port and a second
output port, wherein the first Mach-Zehnder modulating portion is
push-pull driven by a main signal; a second Mach-Zehnder modulating
portion connected to the first output port of the first
Mach-Zehnder modulating portion, wherein the second Mach-Zehnder
modulating portion is push-pull driven by a correction signal; and
an asymmetric light combining portion combining an optical signal
outputted from an output port of the second Mach-Zehnder modulating
portion with an optical signal outputted from the second output
port of the first Mach-Zehnder modulating portion in a light
intensity coupling ratio of r to 1-r in the same polarization
state, wherein an optical path length from the first output port to
the asymmetric light combining portion is substantially equal to an
optical path length from the second output port to the asymmetric
light combining portion.
2. (canceled)
3. The optical modulation circuit according to claim 1, wherein the
light intensity coupling ratio r is 0<r<0.3.
4. The optical modulation circuit according to claim 1, wherein the
correction signal is the same as the main signal or is an inverted
signal of the main signal, and wherein a delay equivalent to a
propagation time of an optical signal to propagate between the
first Mach-Zehnder modulating portion and the second Mach-Zehnder
modulating portion is given between the correction signal and the
main signal.
5. The optical modulation circuit according to claim 1, further
comprising a connecting portion connecting a modulating electrode
of the first Mach-Zehnder modulating portion and a modulating
electrode of the second Mach-Zehnder modulating portion, wherein a
signal propagation delay due to the connecting portion is equal to
a propagation time of an optical signal to propagate between the
first Mach-Zehnder modulating portion and the second Mach-Zehnder
modulating portion.
6. The optical modulation circuit according to claim 5, wherein one
of the modulating electrode of the first Mach-Zehnder modulating
portion and the modulating electrode of the second Mach-Zehnder
modulating portion which is farther from an electrical input is
longer than the modulating electrode closer to the electrical
input.
7. An optical IQ modulation circuit, comprising: two optical
modulation circuits according to claim 1 arranged in parallel; a
light splitting portion splitting input light from an input port to
input the split beams into the two optical modulation circuits; a
light combining portion combining output light beams from the two
optical modulation circuits; and a phase adjustment portion
provided on an optical path from the light splitting portion to the
light combining portion, wherein the phase adjustment portion
adjusts optical phases so that the output light beams from the two
optical modulation circuits are combined with an optical phase
difference of .pi./2 by the light combining portion.
8. A polarization multiplexing IQ modulation circuit, comprising:
two optical IQ modulation circuits according to claim 7 arranged in
parallel; a light splitting portion which splits input light from
an input port to input the split beams into the two optical IQ
modulation circuits; a polarization rotator rotating polarization
of an output light beam from a first optical IQ modulation circuit
among the two optical IQ modulation circuits by 90 degrees; and a
polarization combining portion orthogonal polarization-multiplexing
the output light beam from the first optical IQ modulation circuit
with the polarization rotated by the polarization rotator and an
output light beam from the second optical IQ modulation circuit to
output to the output port as a polarization multiplexed signal.
9. An optical modulation circuit, comprising: a first Mach-Zehnder
modulating portion including a first input port and a second input
port, wherein first Mach-Zehnder modulating portion is push-pull
driven by a main signal; a second Mach-Zehnder modulating portion
connected to the first input port of the first Mach-Zehnder
modulating portion, wherein the second Mach-Zehnder modulating
portion is push-pull driven by a correction signal; and an
asymmetric light combining portion splitting an input optical
signal in a light intensity splitting ratio of r to 1-r to an input
port of the second Mach-Zehnder modulating portion and the second
input port of the first Mach-Zehnder modulating portion, wherein an
optical path length from the asymmetric light splitting portion to
the first input port is substantially equal to an optical path
length from the asymmetric light splitting portion to the second
input port.
10. The optical modulation circuit according to claim 9, wherein
the light intensity coupling ratio r is 0<r<0.3.
11. The optical modulation circuit according to claim 9, wherein
the correction signal is the same as the main signal or is an
inverted signal of the main signal, and wherein a delay equivalent
to a propagation time of an optical signal to propagate between the
first Mach-Zehnder modulating portion and the second Mach-Zehnder
modulating portion is given between the correction signal and the
main signal.
12. The optical modulation circuit according to claim 9, further
comprising a connecting portion connecting a modulating electrode
of the first Mach-Zehnder modulating portion and a modulating
electrode of the second Mach-Zehnder modulating portion, wherein a
signal propagation delay due to the connecting portion is equal to
a propagation time of an optical signal to propagate between the
first Mach-Zehnder modulating portion and the second Mach-Zehnder
modulating portion.
13. The optical modulation circuit according to claim 12, wherein
one of the modulating electrode of the first Mach-Zehnder
modulating portion and the modulating electrode of the second
Mach-Zehnder modulating portion which is farther from an electrical
input is longer than the modulating electrode closer to the
electrical input.
14. An optical IQ modulation circuit, comprising: two optical
modulation circuits according to claim 9 arranged in parallel; a
light splitting portion splitting input light from an input port to
input the split beams into the two optical modulation circuits; a
light combining portion combining output light beams from the two
optical modulation circuits; and a phase adjustment portion
provided on an optical path from the light splitting portion to the
light combining portion, wherein the phase adjustment portion
adjusts optical phases so that the output light beams from the two
optical modulation circuits are combined with an optical phase
difference of .pi./2 by the light combining portion.
15. A polarization multiplexing IQ modulation circuit, comprising:
two optical IQ modulation circuits according to claim 14 arranged
in parallel; a light splitting portion which splits input light
from an input port to input the split beams into the two optical IQ
modulation circuits; a polarization rotator rotating polarization
of an output light beam from a first optical IQ modulation circuit
among the two optical IQ modulation circuits by 90 degrees; and a
polarization combining portion orthogonal polarization-multiplexing
the output light beam from the first optical IQ modulation circuit
with the polarization rotated by the polarization rotator and an
output light beam from the second optical IQ modulation circuit to
output to the output port as a polarization multiplexed signal.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical modulation
circuit applicable to optical communication systems.
BACKGROUND ART
[0002] In order to increase the use efficiency of light spectra,
multilevel modulation such as QAM (quadrature amplitude modulation)
and OFDM (orthogonal frequency division multiplexing) is being
variously examined.
[0003] One of the methods for obtaining a multilevel optical signal
is to drive a push-pull drive Mach-Zehnder modulator (MZM) using a
multilevel electric signal.
[0004] FIG. 1 illustrates a conventional push-pull drive MZM 100.
The example illustrated herein is a single-ended electrode type MZM
using an X-cut lithium niobate (LiNbO.sub.3) substrate. In FIG. 1,
the MZM 100 includes: a Mach-Zehnder interferometer-type optical
circuit including a light splitting portion 101 and a light
combining portion 102; a traveling wave-type modulating electrode
103; and a DC bias electrode 104 of lumped parameter-type. For
simplifying the drawing, each electrode is illustrated with only a
signal line, and the ground electrode is not illustrated. An
optical signal propagating each optical waveguide is given phase
shifts of +.phi. and -.phi. by a driving electric signal inputted
to the modulating electrode 103. Herein, .phi.=(.pi./2V.sub..pi.)V
where V is a voltage level of the driving electric signal and
V.sub..pi. is a voltage to change the relative optical phase
between the arms by .pi.. The optical signal propagating each
optical waveguide is further given a phase difference of .pi. by
bias voltage applied by the DC bias electrode 104. Herein, the MZM
light electric-field response is represented by sin .phi..
[0005] FIG. 2 illustrates a response curve of the electric field of
the output optical signal to the driving voltage in the
conventional MZM. As shown in FIG. 2, in the conventional MZM, the
response curve with respect to the driving voltage is non-linear.
Accordingly, when the MZM is driven by a multilevel electric
signal, the output optical signal is shifted from an ideal and
equal interval output optical signal obtained when the response
curve is linear.
[0006] On the other hand, if the amplitude of the driving voltage
is reduced from 2V.sub..pi. in order to reduce the signal
distortion, large optical loss is generated as illustrated in FIG.
3 (see NPL 1).
CITATION LIST
Non-Patent Literature
[0007] NPL 1: Shogo Yamanaka, Takayuki Kobayashi, Akihide Sano,
Hiroji Masuda, Eiji Yoshida, Yutaka Miyamoto, Tadao Nakagawa,
Munehiko Nagatani, Hideyuki Nosaka, "11.times.171 Gb/s PDM 16-QAM
Transmission over 1440 km with a Spectral Efficiency of 6.4 b/s/Hz
using High-Speed DAC", ECOC 2010, 2010, We. 8. C. 1 [0008] NPL 2:
K. Jinguji, N. Takato, A. Sugita, and M. Kawachi, "Mach-Zehnder
interferometer type optical waveguide coupler with
wavelength-flattened coupling ratio", Electron. Letters, 1990, Vol.
26, No. 17, pp. 1326-1327
SUMMARY OF INVENTION
Technical Problem
[0009] The present invention has been made in the light of the
aforementioned problems, and an object of the present invention is
to provide an optical modulation circuit which suppresses the
non-linearity of light electric-field response.
Solution to Problem
[0010] In order to achieve the aforementioned object, an optical
modulation circuit according to a first aspect of the present
invention includes: a first Mach-Zehnder modulating portion
including a first output port and a second output port, wherein the
first Mach-Zehnder modulating portion is push-pull driven by a main
signal; a second Mach-Zehnder modulating portion connected to the
first output port of the first Mach-Zehnder modulating portion,
wherein the second Mach-Zehnder modulating portion is push-pull
driven by a correction signal; and an asymmetric light combining
portion combining an optical signal outputted from an output port
of the second Mach-Zehnder modulating portion with an optical
signal outputted from the second output port of the first
Mach-Zehnder modulating portion in a light intensity coupling ratio
of r to 1-r, wherein an optical path length from the first output
port to the asymmetric light combining portion is substantially
equal to an optical path length from the second output port to the
asymmetric light combining portion.
[0011] In order to achieve the aforementioned object, moreover, an
optical modulation circuit according to a second aspect of the
preset invention includes: a first Mach-Zehnder modulating portion
including a first input port and a second input port, wherein first
Mach-Zehnder modulating portion is push-pull driven by a main
signal; a second Mach-Zehnder modulating portion connected to the
first input port of the first Mach-Zehnder modulating portion,
wherein the second Mach-Zehnder modulating portion is push-pull
driven by a correction signal; and an asymmetric light combining
portion splitting an input optical signal in a light intensity
splitting ratio of r to 1-r to an input port of the second
Mach-Zehnder modulating portion and the second input port of the
first Mach-Zehnder modulating portion, wherein an optical path
length from the asymmetric light splitting portion to the first
input port is substantially equal to an optical path length from
the asymmetric light splitting portion to the second input
port.
[0012] In an optical modulation circuit according to a third aspect
of the present invention, wherein the light intensity coupling
ratio r is 0<r<0.3.
[0013] In an optical modulation circuit according to a fourth
aspect of the present invention, wherein the correction signal is
the same as the main signal or is an inverted signal of the main
signal, and wherein a delay equivalent to a propagation time of an
optical signal to propagate between the first Mach-Zehnder
modulating portion and the second Mach-Zehnder modulating portion
is given between the correction signal and the main signal.
[0014] An optical modulation circuit according to a fifth aspect of
the present invention further includes: a connecting portion
connecting a modulating electrode of the first Mach-Zehnder
modulating portion and a modulating electrode of the second
Mach-Zehnder modulating portion, wherein a signal propagation delay
due to the connecting portion is equal to a propagation time of an
optical signal to propagate between the first Mach-Zehnder
modulating portion and the second Mach-Zehnder modulating
portion.
[0015] In an optical modulation circuit according to a sixth aspect
of the present invention, wherein one of the modulating electrode
of the first Mach-Zehnder modulating portion and the modulating
electrode of the second Mach-Zehnder modulating portion which is
farther from an electrical input is longer than the modulating
electrode closer to the electrical input.
[0016] An optical modulation circuit according to a seventh aspect
of the present invention includes: two optical modulation circuits
according to claim 1 or 2 arranged in parallel; a light splitting
portion splitting input light from an input port to input the split
beams into the two optical modulation circuits; a light combining
portion combining output light beams from the two optical
modulation circuits; and a phase adjustment portion provided on an
optical path from the light splitting portion to the light
combining portion, wherein the phase adjustment portion adjusts
optical phases so that the output light beams from the two optical
modulation circuits are combined with an optical phase difference
of .pi./2 by the light combining portion.
[0017] A polarization multiplexing IQ modulation circuit according
to an eighth aspect of the present invention includes: two optical
IQ modulation circuits according to claim 7 arranged in parallel; a
light splitting portion which splits input light from an input port
to input the split beams into the two optical IQ modulation
circuits; a polarization rotator rotating polarization of an output
light beam from a first optical IQ modulation circuit among the two
optical IQ modulation circuits by 90 degrees; and a polarization
combining portion orthogonal polarization-multiplexing the output
light beam from the first optical IQ modulation circuit with the
polarization rotated by the polarization rotator and an output
light beam from the second optical IQ modulation circuit to output
to the output port as a polarization multiplexed signal.
Advantageous Effects of Invention
[0018] According to the present invention, it is possible to
provide an optical modulation circuit having response
characteristics with the non-linearity reduced by generating a
secondary component in the light electric-field response to the
driving voltage to add the same to the primary component.
BRIEF DESCRIPTION OF DRAWINGS
[0019] FIG. 1 is a configuration diagram illustrating the
configuration of a conventional MZM;
[0020] FIG. 2 is a diagram for explaining signal distortion caused
in the conventional MZM;
[0021] FIG. 3 is a diagram for explaining optical loss caused in
the conventional MZM;
[0022] FIG. 4 is a configuration diagram illustrating the
configuration of an optical modulation circuit according to a first
embodiment of the present invention;
[0023] FIG. 5 is a diagram showing a response curve obtained by the
optical modulation circuit illustrated in FIG. 4;
[0024] FIG. 6 is a diagram showing response curves which are
obtained by plotting the first term of the right-hand side of
Equation 2, the second term of the right-hand side thereof, and T
of the left-hand side with respect to V.sub.1/V.sub..pi.1
(=2.phi./.pi.);
[0025] FIG. 7A is a diagram showing an output optical signal
spectrum when the optical modulation circuit is driven with a sine
wave with a whole amplitude of 2V.sub..pi.;
[0026] FIG. 7B is a diagram showing an output optical signal
spectrum when the optical modulation circuit is driven with a sine
wave with a whole amplitude of 2V.sub..pi.;
[0027] FIG. 8 is a diagram showing r and a dependences of SFDR
obtained by Equation 4;
[0028] FIG. 9 is a diagram showing r and a dependence of
theoretical optical loss obtained by Equation 5;
[0029] FIG. 10 is a configuration diagram illustrating the
configuration of an optical modulation circuit according to a
second embodiment of the present invention;
[0030] FIG. 11 is a configuration diagram illustrating the
configuration of an optical modulation circuit according to a third
embodiment of the present invention; and
[0031] FIG. 12 is a configuration diagram illustrating the
configuration of an optical modulation circuit according to a
fourth embodiment of the present invention.
DESCRIPTION OF EMBODIMENTS
[0032] The present invention relates to a circuit configuration of
modulation circuits, and the effect thereof is independent of the
materials constituting the modulation circuits. In the embodiments
shown below, the materials constituting the modulation circuits are
not particularly specified. The materials constituting the
modulation circuits can be multicomponent oxide crystals with the
Pockels effect, which is a kind of electro-optic (EO) effects, such
as LiNbO.sub.3 (LN), KTa.sub.1-xNb.sub.xO.sub.3,
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3, GaAs-based or InP-based
compound semiconductors capable of implementing refractive-index
modulation by the Pockels effect and quantum confined stark effect
(QCSE), and polymers with an EO effect, such as chromophore.
Moreover, for the purpose of manufacturing a modulation circuit
having a complicated configuration with a low loss, the optical
modulation circuit may have a joint structure of different types of
substrates including a substrate made of one of the aforementioned
materials and a silica-based planar lightwave circuit (PLC).
[0033] The effects of the present invention can be obtained in any
case where the modulating electrode of the Mach-Zehnder modulating
portion is single-ended type or differential type. As generally
well known, the arrangement of the modulating electrode in the
push-pull driving Mach-Zehnder modulation circuit depends on the
type of the substrate, the direction of the crystal axis thereof,
and the like. Generally, the single-ended type is used in X-cut LN
substrates, and the differential type is used in Z-cut LN
substrates (even in Z-cut LN substrates, the modulating electrode
can be configured as the single-ended type by using polarization
reversal), for example. Normally, the signal electrode of the
single-ended type is placed in the middle between the optical
waveguide arms, and the signal electrodes of the differential type
are placed just above the respective arms (in the case of the
single-ended type using a polarization-reversed Z-cut LN substrate,
the signal electrode is placed just above one of the arms). In the
optical modulation circuit according to the embodiments shown in
the examples below, a description is given by basically assuming
the single-ended type for simplification of the drawings. However,
even in the case of using the differential-type, the response
characteristic of the Mach-Zehnder modulating portion reduces to
the same mathematical expression as that of the single-ended type.
Accordingly, the choice of the electrode arrangement cannot
influence the effect of the present invention. The drawings
concerning the embodiments shown below by way of example illustrate
the signal electrodes but not ground electrodes for
simplification.
[0034] In the optical modulation circuit according to each
embodiment illustrated below by way of example, the both arms in
the Mach-Zehnder modulating portion are designed to have a same
optical path length. The optical path lengths of the arms have some
differences therebetween in practice because of process errors, DC
drifts, and the like, but generally, such differences are
compensated by adjusting the DC bias. The amount of compensation
varies depending on the materials, manufacturing conditions, use
environments of the modulator, and the like and cannot be uniquely
decided. Accordingly, in the following embodiments, the phase
difference between the arms given by the DC bias does not include
the compensation of the optical path lengths.
First Embodiment
[0035] FIG. 4 illustrates an optical modulation circuit 400
according to a first embodiment of the present invention.
[0036] In FIG. 4, the optical modulation circuit 400 includes a
main input port 401, first and second Mach-Zehnder modulating
portions 410 and 420, asymmetric light combining portion 407, and a
main output port 402. The first Mach-Zehnder modulating portion 410
has a 2-output cross-bar switch configuration using directional
couplers 411 and 422 as input-side and output-side couplers,
respectively. A cross-side output port 416 is connected to the
asymmetric light combining portion 407. A bar-side output port 415
is connected to the second Mach-Zehnder modulating portion 420. The
second Mach-Zehnder modulating portion 420 has a 1-input 1-output
configuration using Y couplers 421 and 422 as the input-side and
output-side couplers, respectively. The Mach-Zehnder modulating
portions 410 and 420 include traveling-wave type modulating
electrodes 413 and 423 and lumped-parameter type DC-bias electrodes
414 and 424, respectively. Furthermore, the optical modulation
circuit 400 separately includes a DC bias electrode 404 for
adjusting the relative phase of the optical signals inputted into
the asymmetric light combining portion 407. In the Mach-Zehnder
modulating portions 410 and 420, the DC bias electrodes 414 and 424
are used to adjust the phases so that the phase difference between
the arms is .pi. when the voltage of the driving signal is zero.
The asymmetric light combining portion 407 may be composed of an
asymmetric coupler with the coupling ratio fixed but may be
conveniently a variable coupler whose coupling ratio is adjustable
for flexible adjustment. The optical path length from the bar-side
output port 415 of the first Mach-Zehnder modulating portion 410
through the second Mach-Zehnder modulating portion 420 to the
asymmetric light combining portion 407 is the same as that of the
optical path from the cross-side output port 416 of the first
Mach-Zehnder modulating portion 410 to the asymmetric light
combining portion 407.
[0037] In addition to the main output port 402, a tap circuit and a
monitor output port may be properly arranged to monitor the signal
state in the middle of the circuit. The tap circuit can be placed
at the two output ports of the first Mach-Zehnder modulating
portion 410, the output ports of the second Mach-Zehnder modulating
circuit 420, and the like.
[0038] Herein, the light electric-field responses at the cross-side
and bar-side of the first Mach-Zehnder modulating portion 410 are
indicated by T.sub.1c and T.sub.1b, respectively. The light
electric-field response of the second Mach-Zehnder modulating
portion 420 is indicated by T.sub.2. The light electric-field
response of the entire modulation circuit is indicated by T.
T.sub.1c, T.sub.1b, T.sub.2, and T are expressed by Equation 1
below. Herein, the non-essential phase coefficient is omitted.
[ Equation 1 ] T 1 c = sin ( .phi. 1 ) = sin ( .pi. 2 V .pi. 1 V 1
) T 1 b = cos ( .phi. 1 ) = cos ( .pi. 2 V .pi. 1 V 1 ) T 2 = sin (
.phi. 2 ) = sin ( .pi. 2 V .pi. 2 V 2 ) T = 1 - r T 1 c + j .theta.
r T 2 T 1 b ( 1 ) ##EQU00001##
[0039] Constants V.sub..pi.1 and V.sub..pi.2 denote voltages
(constants) that change inter-arm relative optical phases by .pi.
in the Mach-Zehnder modulating portions 410 and 420, respectively.
Variables V.sub.1 and V.sub.2 denote driving signal voltages
inputted to the Mach-Zehnder modulating portions 410 and 420,
respectively. .theta. denotes the inter-arm phase difference given
by the DC bias electrode 404. Moreover, r denotes a light intensity
coupling ratio in the asymmetric light combining portion 407.
Herein, light coupling intensity of an input from the second
Mach-Zehnder modulating portion 402/light coupling intensity of an
input from the cross-side output port 416 of the first Mach-Zehnder
modulating portion 410 is set to r/1-r. For the above-described
equation is of the light electric-field responses, the square roots
of r and 1-r are included as coefficients of the respective
terms.
[0040] V.sub.2 needs to be inputted to the modulating electrode
with a certain delay with respect to V.sub.1. This is because it
takes a certain period of time for optical signal modulated by the
first Mach-Zehnder modulating portion 410 to reach the second
Mach-Zehnder modulating portion 420 and the driving electric signal
V.sub.2 needs to be delayed accordingly. To be specific, time delay
.tau. of V.sub.2 with respect to V.sub.1 needs to be .tau.=NL/c.
Herein, L is physical length of the optical waveguide between the
point where the interaction between the optical signal and
electrical signal starts in the modulating electrode 413, into
which V.sub.1 is inputted, and the point where the interaction
between the optical signal and electrical signal starts in the
modulating electrode 423, into which V.sub.2 is inputted. N is
group velocity of optical signal in the same optical waveguide, and
c is light velocity.
[0041] In order to obtain a response with high linearity as the
final response T, there are two driving methods. One of the methods
is to set correction signal V.sub.2 equal to inverted signal of
main signal V.sub.1 (V.sub.2=-V.sub.1) and set .theta.=0, and the
other is to set the correction signal V.sub.2 equal to the main
signal V.sub.1 (V.sub.2=V.sub.1) and set .theta.=.pi.. In either
method, the response T of the entire modulation circuit is
expressed by Equation 2 below.
[ Equation 2 ] T = 1 - r sin ( .phi. ) - r cos ( .phi. ) sin (
.phi. ) = 1 - r sin ( .phi. ) - r 2 sin ( 2 .phi. ) ( 2 )
##EQU00002##
[0042] Herein, .phi.=(.pi./2V.sub..pi.1).about.V.sub.1, and it is
assumed that V.sub..pi.2=V.sub..pi.1. In the case of
V.sub.2=V.sub.1, the same response can be obtained by setting
.theta.=0 instead of 0=.pi. and using the DC bias electrode 424 to
adjust the bias voltage so that the inter-arm phase difference of
the second Mach-Zehnder modulating portion 420 is -.pi. when the
driving signal voltage is zero.
[0043] FIG. 5 shows a response curve obtained by plotting the
values of T relative to V.sub.1/V.sub..pi.1 (=2.phi./.pi.) when
r=0.12 in Equation 2. The responses of a conventional MZM
illustrated in FIGS. 2 and 3 are sine functions while the response
of the first embodiment shown in FIG. 5 is closer to a straight
line and is increased in linearity. Moreover, the optical loss is
very little.
[0044] FIG. 6 shows response curves obtained by plotting the first
term of the right-hand side of Equation 2, the second term of the
right-hand side thereof, and T of the left-hand side thereof
relative to V.sub.1/V.sub..pi.1 (=2.phi./.pi.). As shown in FIG. 6,
the first term of the right-hand side is a sinusoidal response term
similar to the conventional MZM but is added with a sinusoidal
response term as the second term which has a response period half
of that of the first term and has an opposite sign to the first
term. The second term suppresses the non-linearity of the first
term, and the response T of the entire modulation circuit is close
to a straight line, showing that the linearity thereof is
increased.
[0045] Hereinafter, in order to qualify the linearity of the
response, a description is given of an output optical signal
spectrum in the case of driving a modulator with a pure sine
signal. In an ideal linear response modulator, the output light
electric-field is proportional to the driving signal. Accordingly,
the output optical signal spectrum is supposed to include only the
fundamental wave components of f.sub.0.+-.f.sub.s for optical
carrier frequency f.sub.0 where f.sub.s is the frequency of the
driving sine wave. However, because the response of an actual
modulator includes non-linearity, the output optical signal
spectrum thereof includes also harmonic components of
f.sub.0.+-.nf.sub.s (n is a natural number not less than 2). The
intensity ratio of the fundamental wave component to the maximum
harmonic component is called the spurious-free dynamic range (SFDR)
and can be used an index of the linearity.
[0046] FIGS. 7A and 7B show output optical signal spectra when the
conventional MZM illustrated in FIG. 1 and the optical modulation
circuit according to the first embodiment illustrated in FIG. 4
with r=0.12 are individually driven by a sine wave with a whole
amplitude of 2V.sub..pi.. In FIGS. 7A and 7B, the horizontal axis
represents the harmonic order n, and the vertical axis represents
the spectrum intensity. As shown in FIG. 7A, in the conventional
MZM, the intensity of the third harmonics is high, and the SFDR is
about 18.3 dB. On the other hand, as shown in FIG. 7B, in the
optical modulation circuit according to the first embodiment with
r=0.12, the third and fifth harmonics have substantially a same
intensity, and the SFDR is about 36.8 dB. Accordingly, the SFDR of
the optical modulation circuit according to the first embodiment
has an improvement of 18.5 dB over the conventional example.
[0047] The SFDR depends on the amplitude of the driving sine wave
and the value of r. For explaining the dependence, the electric
field E.sub.out of the output optical signal of the optical
modulation circuit according to the first embodiment is developed
using the Bessel function of the first kind J as follows. Herein,
the driving signal is a sine wave with a whole amplitude of
2.alpha.V.sub..pi. (.alpha. is a modulation index) and an angular
frequency .omega. (=2.pi.f.sub.s).
[ Equation 3 ] E out = 1 - r sin ( .pi. 2 V .pi. .alpha. V .pi. sin
( .omega. t ) ) - r 2 sin ( .pi. V .pi. .alpha. V .pi. sin (
.omega. t ) ) = 2 m = 0 .infin. { 1 - r J 2 m + 1 ( .pi. .alpha. 2
) - r 2 J 2 m + 1 ( .pi. .alpha. ) } sin { ( 2 m + 1 ) .omega. t }
( 3 ) ##EQU00003##
[0048] The SFDR can be obtained as a ratio of the square of the
term with m=0 (the fundamental) to the square of the coefficient of
the term with m>0 (harmonic) as follows.
[ Equation 4 ] SFDR dB ( .alpha. , r ) = 20 log 1 - r J 1 ( .pi.
.alpha. 2 ) - r 2 J 1 ( .pi. .alpha. ) MAX m [ 1 - r J 2 m + 1 (
.pi..alpha. 2 ) - r 2 J 2 m + 1 ( .pi. .alpha. ) ] ( 4 )
##EQU00004##
[0049] Equation 4 above expresses the SFDR of the output optical
signal from the optical modulation circuit according to the first
embodiment illustrated in FIG. 4. However, the SFDR of the
conventional MZM illustrated in FIG. 1 can be calculated by setting
r=0 in Equation 4 because the response of the conventional MZM
illustrated in FIG. 1 is the same as the response of the optical
modulation circuit according to the first embodiment with r=0.
[0050] FIG. 8 shows a contour plot of the values of the SFDR
obtained by Equation 4 with respect to .alpha. on the horizontal
axis and r on the vertical axis. As shown in FIG. 8, the optimal
value of r (the value of r maximizing the SFDR) changes gradually
with .alpha.. For example, r=0.12 (the condition in FIG. 7B) is
optimal when .alpha.=1, and the SFDR is 36.8 db. When the driving
amplitude is reduced to .alpha.=0.5, r=0.07 is optimal, and the
SFDR is 61.5 dB.
[0051] Generally, as the driving amplitude is attenuated, the
linearity is increased, and the SFDR is increased. However, the
theoretical optical loss increases when the driving amplitude is
attenuated even in the optical modulation circuit according to the
first embodiment in similar manner to when the driving amplitude is
attenuated in the conventional MZM as shown in FIG. 3. Herein, the
theoretical optical loss refers to an optical loss for the peak
voltage of the driving signal. To be specific, the theoretical
optical loss is expressed by the following equation.
[ Equation 5 ] Loss dB ( .alpha. , r ) = - 20 log { 1 - r sin (
.pi..alpha. 2 ) - r 2 sin ( .pi. .alpha. ) } ( 5 ) ##EQU00005##
[0052] FIG. 9 shows a contour plot of values of the theoretical
optical loss obtained by Equation 5 with respect to .alpha. on the
horizontal axis and r on the vertical axis. The theoretical optical
loss remains as small as 0.56 dB where .alpha.=1 and r=0.12 (the
conditions of FIG. 7B, SFDR=36.8 dB). For providing the comparable
SFDR by the conventional technique (corresponding to r=0), it is
necessary to narrow the driving amplitude to .alpha.=0.37. At this
time, the theoretical optical loss is 5.21 dB. In other words, if
it is necessary that the SFDR is 36.8 dB, the theoretical optical
loss of the optical modulation circuit according to the first
embodiment can be 4.65 dB lower than that of the conventional
technique.
[0053] As revealed from FIG. 9, the theoretical optical loss
depends on .alpha. more than r and is minimized in a range of
.alpha. from 1.0 to 1.4. On the other hand, as shown in FIG. 8, the
SFDR is increased as .alpha. is reduced. Specifically, the range of
.alpha.>1.4 is disadvantageous in terms of both the optical loss
and SFDR. In the range of .alpha.>1.4, the optical loss and SFDR
substantially trade off each other. Accordingly, the setting range
of .alpha. is suitably 0<.alpha.<1.4. Moreover, as shown in
FIGS. 8 and 9, the range of r where the SFDR is minimized is
located in a range of r<0.3 in 0<.alpha.<1.4, and the
theoretical optical loss is reduced as r is reduced. Accordingly,
the setting region of r is suitably 0<r<0.3.
[0054] The output coupler 412 of the Mach-Zehnder modulating
portion 410 can be a multi-mode interference (MMI) coupler or a
wavelength insensitive coupler (WIN) shown in NPL 2 besides the
directional coupler. Optical signals from the output ports of any
2-input 2-output coupler are inverted to each other. Accordingly,
Equation 1 can be established if the phase adjustment is properly
performed using the bias electrode 414. This can be introduced from
the reciprocity of optical couplers and the law of conservation of
energy (to be strict, the reciprocity could be lost in some cases
because of the internal loss of couplers, but is not a problem if
couplers with an internal loss small enough are employed).
Moreover, the input coupler 411 may be either a 2-input 2-output
coupler illustrated in FIG. 4 by way of example or a Y coupler. In
the case of using a Y coupler, which is not called a cross-bar
switch type, all Equations 1 to 5 described above can be
established if the phase difference between the arms is adjusted by
the DC bias electrode 414 so that the output light to the
cross-side output port 416 is minimized when the driving signal
voltage is 0. Accordingly, there is no essential difference between
use of the Y coupler and use of the 2-input 2-output coupler. In a
similar manner, the couplers 421 and 422 may be individually
composed of a 2-input 2-output coupler.
Second Embodiment
[0055] FIG. 10 illustrates an optical modulation circuit 1000
according to a second embodiment of the present invention.
[0056] In FIG. 10, the optical modulation circuit 1000 according to
the second embodiment of the present invention is the same as the
optical modulation circuit 400 of the first embodiment shown in
FIG. 4 excepting that the directions that light is inputted and
outputted are reversed and modulating electrodes 1013 and 1023 are
provided so that the input and output sides thereof are inverted to
those of the modulating electrodes 413 and 423, respectively. The
optical modulation circuit 1000 according to the second embodiment
is a reciprocal passive optical circuit other than the modulating
electrodes and thereby has exactly the same functions as those of
the optical modulation circuit 400 according to the first
embodiment illustrated in FIG. 4. In the optical modulation circuit
400 according to the first embodiment, the driving signal of the
Mach-Zehnder modulating portion 420 needs be delayed with respect
to the driving signal of the Mach-Zehnder modulating portion 410.
Since the optical modulation circuit 1000 according to the second
embodiment has the input and output sides inverted, the driving
signal of the Mach-Zehnder modulating portion 510 needs be delayed
with respect to the driving signal of the Mach-Zehnder modulating
portion 520.
[0057] As for the name of each member, the asymmetric light
combining portion 407 is replaced with an asymmetric light
splitting portion 1007, and the bar-side and cross-side output
ports 415 and 416 are replaced with input and output ports 1015 and
1016 of the Mach-Zehnder modulating section 1010, respectively. The
names of these members are just changed for the input and output
sides are inverted, but the members can be composed of components
having the same physical structures as those of the optical
modulation circuit 400.
Third Embodiment
[0058] FIG. 11 illustrates an optical modulation circuit 1100
according to a third embodiment of the present invention.
[0059] In FIG. 11, the optical modulation circuit 1100 according to
the third embodiment of the present invention differs from the
optical modulation circuit 400 according to the first embodiment
illustrated in FIG. 4 in that a modulating electrode 1113 and a
modulating electrode 1123 are connected by a connecting portion
1133 to allow the optical modulation circuit 400 to be driven with
a single input of driving signal. The other part is the same as
that of the optical modulation circuit 400.
[0060] As described in the first embodiment, the correction signal
V.sub.2 driving a Mach-Zehnder modulating portion 1120 is the same
as or is inverted to the main signal V.sub.1 driving a Mach-Zehnder
modulating portion 1110. By using the configuration of the third
embodiment, V.sub.2 is equal to V.sub.1, and the modulation circuit
needs to include only one input port for the driving signal, so
that electric wiring to drive the modulator can be simplified. The
propagation delay .tau. due to the connecting portion 1133 needs to
be designed so that .tau.=NL/c using the aforementioned N and L.
The correction signal V.sub.2 attenuates because of the propagation
loss of the modulating electrode 1113 and connecting portion 1133
with respect to the main signal V.sub.1, and V.sub..pi.2 needs to
be set smaller than V.sub..pi.1 accordingly. To be specific,
V.sub..pi.2/V.sub..pi.1 is set to (amplitude of V.sub.2)/(amplitude
of V.sub.1) by a correcting method, including a method of setting
the modulating electrode 1123 longer than the modulating electrode
1113. This can provide the response as expressed by Equation 2. As
apparent from the description of the first and second embodiments,
the same effects as those of the third embodiment can be obtained
in the configuration in which the input and output of light and the
input and output of each modulating electrode are individually
inverted in the optical modulation circuit according to the third
embodiment.
Fourth Embodiment
[0061] FIG. 12 illustrates a polarization multiplexing IQ
modulation circuit 1200 according to a fourth embodiment of the
present invention.
[0062] In FIG. 12, the optical modulation circuit 1200 according to
the fourth embodiment of the present invention includes four
optical modulation circuits 400 according to the first embodiment
illustrated in FIG. 4 in parallel to constitute a polarization
multiplexing IQ modulation circuit. Light inputted to a main input
port 1201 is split into four by alight splitting portion 1203 to be
inputted to high-linearity modulation circuits 1211 to 1214 each
having the same configuration as the optical modulation circuit
according to the first embodiment illustrated in FIG. 4. After the
phases of the optical signals outputted from the high-linearity
modulation circuits 1211 and 1212 are adjusted by a DC bias
electrode 1221 so that the relative phase thereof is .pi./2, the
optical signals are then combined. The polarization axis thereof is
then rotated by 90 degrees with a polarization rotator 1231. After
the phases of the optical signals outputted from the high-linearity
modulation circuits 1213 and 1214 are adjusted by a DC bias
electrode 1222 so that the relative phase thereof is .pi./2, the
optical signals are then combined. The both outputted signals are
orthogonal-polarization multiplexed by a polarization combining
portion 1204 to be outputted from the main output port 1202 as a
polarization multiplexed signal.
[0063] In the configuration of the fourth embodiment, each of four
conventional MZMs which are arranged in parallel in a polarization
multiplexing IQ modulator shown in many documents, including NPL1,
is replaced with the high-linearity modulation circuit illustrated
in FIG. 4. The high-linearity modulation circuits 1211 to 1214
correspond to I and Q components of each polarization channel.
Accordingly, the provided polarization multiplexing IQ modulator
has a light electric-field response with the non-linearity reduced.
Each of the high-linearity modulation circuits 1211 to 1214 may be
composed of the optical modulation circuit according to the second
embodiment illustrated in FIG. 10 or the optical modulation circuit
according to the third embodiment illustrated in FIG. 11 instead of
the optical modulation circuits according to the first embodiment
illustrated in FIG. 4.
[0064] Moreover, a single polarization IQ modulation circuit can be
provided in the following manner. In the optical modulation circuit
according to the fourth embodiment illustrated in FIG. 12, the
light splitting portion 1203 is configured to split light into two,
and two adjacent ones of the high-linearity modulation circuits
(high-linearity modulation circuits 1211 and 1212, for example) are
used. The other two circuits (the high-linearity modulation
circuits 1213 and 1214, for example), the polarization rotator
1231, and the polarization combining portion 1204 are
eliminated.
[0065] As described above, according to the present invention, by
generating the secondary component of the response of the light
electric-field with respect to driving voltage and adding the same
to the primary component, it is possible to provide an optical
modulation circuit having response characteristics with the
non-linearity reduced.
REFERENCE SIGNS LIST
[0066] 401, 1001, 1101, 1201: MAIN INPUT PORT [0067] 402, 1002,
1102, 1202: MAIN OUTPUT PORT [0068] 404, 1004, 1104, 1221, 1222: DC
BIAS ELECTRODE [0069] 407, 1117: ASYMMETRIC LIGHT COMBINING PORTION
[0070] 410, 1010, 1110: FIRST MACH-ZEHNDER MODULATING PORTION
[0071] 411, 1011, 1111: INPUT COUPLER OF FIRST MACH-ZEHNDER
MODULATING PORTION [0072] 412, 1012, 1112: OUTPUT COUPLER OF FIRST
MACH-ZEHNDER MODULATING PORTION [0073] 413, 1013, 1113: MODULATING
ELECTRODE OF FIRST MACH-ZEHNDER MODULATING PORTION [0074] 414,
1014, 1114: DC BIAS ELECTRODE OF FIRST MACH-ZEHNDER MODULATING
PORTION [0075] 415, 1115: BAR-SIDE OUTPUT PORT OF FIRST
MACH-ZEHNDER MODULATING PORTION [0076] 416, 1116: CROSS-SIDE OUTPUT
PORT OF FIRST MACH-ZEHNDER MODULATING PORTION [0077] 420, 1020,
1120: SECOND MACH-ZEHNDER MODULATING PORTION [0078] 421, 1021,
1121: INPUT COUPLER OF SECOND MACH-ZEHNDER MODULATING PORTION
[0079] 422, 1022, 1122: OUTPUT COUPLER OF SECOND MACH-ZEHNDER
MODULATING PORTION [0080] 423, 1023, 1123: MODULATING ELECTRODE OF
SECOND MACH-ZEHNDER MODULATING PORTION [0081] 424, 1024, 1124: DC
BIAS ELECTRODE OF SECOND MACH-ZEHNDER MODULATING PORTION [0082]
1015: BAR-SIDE INPUT PORT OF FIRST MACH-ZEHNDER MODULATING PORTION
[0083] 1016: CROSS-SIDE INPUT PORT OF FIRST MACH-ZEHNDER MODULATING
PORTION [0084] 1107: ASYMMETRIC LIGHT SPLITTING PORTION [0085]
1133: CONNECTING PORTION OF MODULATING ELECTRODE [0086] 1203: LIGHT
SPLITTING PORTION [0087] 1204: POLARIZATION COMBINING PORTION
[0088] 1211, 1212, 1213, 1214: HIGH-LINEARITY MODULATION CIRCUIT
[0089] 1231: POLARIZATION ROTATOR
* * * * *