Description
Title of the Invention
RZ OPTICAL MODULATOR AND RZ OPTICAL MODULATION
5 METHOD
Technical field
The present invention relates to an RZ optical modulator and an RZ
optical modulation method which perform transmission using an RZ (return
to zero) code as a transmission code.
10 Background art
More capacity enlargement for a core transmission system is
required along with the increase of the internet traffic. Under such
condition, the coherent optical transmission technology is noted. An
optical transmitter used in a coherent optical transmission phase-modulates
15 an optical signal. And an optical receiver used in a coherent optical
transmission receives a phase-modulated optical signal, mixes the received
optical signal with the local oscillation light, generates a beat signal and
reproduces transmission data from the beat signal.
In the coherent optical transmission technology, in particular, DP-
20 QPSK can improve the wavelength utilization efficiency to double by
performing the QPSK modulation for the orthogonal polarized optical
waves with different signals respectively. For this reason, consideration
for realizing a large capacity optical communication system using DPQPSK
is performed. Incidentally, DP-QPSK is an abbreviation of dual
25 polarization-quadrature phase shift keying (polarization multiplexing with
four phase shift modulation).
In recent years, it has begun to be studied to apply the coherent
optical transmission technology to an inter-continental submarine optical
transmission system. The yield strength to the deterioration of OSNR
(optical signal to noise ratio) caused by a multiple cascade connection of
the transponders is requested for the signal used in a submarine optical
transmission system. For this reason, in a submarine optical transmission
5 system, an RZ (return to zero) code in addition to an NRZ (non return to
zero) code used widely in a land optical transmission system is also used.
And by applying the coherent transmission technology to an RZ code
transmission system, it is expected to improve the transmitter OSNR.
In a coherent optical transmission system using an RZ code, the
10 light which a CW (continuous wave) light source outputs is inputted to an
RZ modulator and an RZ pulse train is generated. The RZ modulator
which generates an RZ pulse train from the CW light is called the RZ
carver. The RZ pulse train generated at the RZ carver is inputted to the
DP-QPSK modulator. The RZ pulse train inputted to the DP-QPSK
15 modulator is separated into two, and QPSK modulation is performed by the
respective different data. QPSK-modulated two RZ pulse trains are
multiplexed so that the polarizations of the modulated light become
orthogonal each other and transmitted to a transmission line.
In relation to the present invention, PTL(patent document) 1
20 discloses a phase modulation communication system which branches light
signals amplitude-modulated by a clock, and performs the polarization
mixing after the phase modulation for the respective light signals by the
different data. PTL 2 discloses an optical transmission apparatus with
which an optical phase modulator and an optical intensity modulator have
25 been connected in series.
Citation List
Patent Literature
[PTL 11 Japanese Patent Unexamined Application No. 2003-249897
[PTL 21 Japanese Patent Unexamined Application No. 2004-088250
Summary of invention
Technical Problem
As mentioned above, the RZ carver for generating an RZ pulse train
5 is arranged between the CW light source and the DP-QPSK modulator in a
coherent optical transmission system using an RZ code.
In general, a CW light source oscillates in only TE (transverse
electric) oscillation mode, and the TEITM (transverse magnetic) ratio
(polarization extinction ratio) is no smaller than 30 dB in a typical CW
10 light source. And in an optical transmission system using an NRZ code,
The CW light source and the DP-QPSK modulator are connected by a
polarization preserving optical fiber without setting up a connection point
on the way. Accordingly, in an optical transmission system using an NRZ
code, the light outputted from a CW light source is inputted to a DP-QPSK
15 modulator while keeping the high polarization extinction ratio.
However, when a part like the RZ carver is inserted between the
CW light source and the DP-QPSK modulator in order to use an RZ code, a
connection point arises to a polarization preserving optical fiber. A
polarization axis of an optical fiber needs to be made coincided in addition
20 to the center axis of the core on connecting the polarization preserving
optical fiber. However, it is difficult to make these axes coincided and
the polarization extinction ratio degrades by incompleteness of the axis
adjustment. For example, when a component such as the RZ carver is
inserted between the CW light source whose polarization extinction ratio is
25 about 30 dB and the DP-QPSK modulator, the polarization extinction ratio
of the RZ pulse train inputted to the DP-QPSK modulator may degrade to
about 10 dB.
On the other hand, the DP-QPSK modulator is generally produced
as an optical waveguide device. Additionally, DP-QPSK modulator is
often designed with supposing that either linearly polarized light of TE or
TM is inputted. Therefore, only when the light inputted from the CW
light source to the DP-QPSK modulator has the sufficient polarization
extinction ratio, the OSNR of the transmission signal outputted from the
DP-QPSK modulator is reserved.
However, when the RZ carver is arranged between the CW light
source and the DP-QPSK modulator, the polarization extinction ratio of the
light inputted to the DP-QPSK modulator degrades to modulate the DPQPSK
with the RZ code. For this reason, when the DP-QPSK modulation
method is tried to be applied to an optical transmission system using RZ
code, the OSNR of the transmission signal may degrade.
Although disclosing a general phase modulation communication
system, PTL 1 does not disclose the technology which settles the problem
that the polarized wave extinction ratio of the light signal degrades by
using the RZ carver. Although PTL 2 discloses a configuration in which
the RZ code is performed with the optical phase modulation, the
polarization extinction ratio of the light signal which is also inputted to the
optical phase modulator degrades by an optical intensity modulator
disclosed in PTL 2. And PTL 2 does not disclose the configuration for
settling the problem that the polarization extinction ratio degrades.
That is, when the DP-QPSK modulation method is applied to an
optical transmission system using RZ code, neither PTL 1 nor PTL 2,
describes a means for settling the problem that the OSNR of the
transmission signal may degrade by the polarization extinction ratio
degradation.
The object of the present invention is to provide the technology for
improving the quality of the modulated transmission signal in an optical
transmission system using RZ code.
Solution to Problem
An RZ optical modulator of the present invention includes: an RZ
carver for outputting an optical pulse train of an RZ (return to zero) code;
a polarization adjustment means for improving a polarization extinction
ratio of the optical pulse train outputted from the RZ carver and outputting
the optical pulse train whose extinction ratio is improved; and an optical
modulation means for modulating and outputting the optical pulse train
outputted from the polarization adjustment means.
An RZ modulation method of the present invention includes:
outputting an optical pulse train of RZ code; improving a polarization
extinction ratio of the optical pulse train; and modulating the optical pulse
train whose polarization extinction ratio is improved and outputting the
modulated optical pulse train.
Advantageous Effects of Invention
In an optical transmission system using an RZ code, the present
invention offers the effect that the quality of the transmission signal is
improved.
Brief Description of Drawings
Fig. 1 shows a composition of an optical transmitter of the first
exemplary embodiment.
Fig. 2 shows a constellation just after the modulation of X-I signal.
Fig. 3 shows a constellation just after the modulation of X-Q signal.
Fig. 4 shows a constellation of the mixed signal of X-I signal and
X-Q signal.
Fig. 5 shows a constellation of the polarization-combined light with
PBC.
Fig. 6 shows a composition of the optical modulator of the second
exemplary embodiment.
Fig. 7 shows a composition of the optical modulator of the third
exemplary embodiment.
Description of Embodiments
In each of the first to the third exemplary embodiments below the
exemplary embodiment in which an RZ optical modulator of the present
invention is applied to an optical transmitter.
[The first exemplary embodiment]
Fig. 1 is a figure which shows a composition of the optical
transmitter of the first exemplary embodiment of the present invention.
An optical transmitter 1 shown in Fig. 1 includes a CW light source 2, an
RZ carver 3, a polarizer 4 and DP-QPSK modulator 5. The polarizer 4 is
arranged between the RZ carver 3 and the DP-QPSK modulator 5. And
the CW light source 2, the RZ carver 3, the polarizer 4 and DP-QPSK
modulator 5 are connected each other with polarization maintaining fibers
respectively.
The CW light source 2 oscillates the CW light which becomes a
carrier of a transmission signal and outputs the CW light to the RZ carver 3.
For example, the polarized wave extinction ratio of the light outputted
from the CW light source 2 is about 30 dB. The RZ carver 3 converts the
inputted CW light into a pulse train of an RZ code. For example, an RZ
pulse train can be generated from the CW light by the amplitude
modulation for the CW light with a clock whose rate is equal to the baud
rate (the number of modulation times per second) of the phase modulation.
Here, by passing through the RZ carver 3, the polarization extinction ratio
of the RZ pulse degrades into about 1 OdB.
The RZ pulse train generated at the RZ carver 3 passes through the
polarizer 4, and is inputted to the DP-QPSK modulator 5. The polarizer 4
outputs the light whose polarization extinction ratio is no more than lOdB
as a linearly polarized light by whose polarization extinction ratio is high.
The polarizer 4 is an optical component used for a polarization adjustment
which is also called a polarizer. According to the first exemplary
embodiment, the RZ pulse train which has passed through the polarizer 4 is
inputted to the DP-QPSK modulator 5 with the polarization extinction ratio
no smaller than 20 dB. Accordingly, by setting the polarization direction
of the output of the polarizer 4 to the direction fitting to the coupling with
the DP-QPSK modulator 5, it is enabled to operate the DP-QPSK modulator
5 by the RZ pulse train with the high extinction ratio. For example, when
the DP-QPSK modulator 5 is designed by the condition that the electric
field of the incident light is in the direction parallel to the substrate, the
polarized light of the light outputted from the polarizer 4 and inputted to
the DP-QPSK modulator 5 shall be made in the direction parallel to the
substrate of the DP-QPSK modulator 5. Further, a part for the
polarization adjustments which improves and outputs the polarization
extinction ratio of the inputted light may be used instead of the polarizer.
The DP-QPSK modulator 5 is an optical waveguide device
configured as two QPSK modulators are connected in parallel. A PBS 51
separates the inputted light in two lights X and Y whose polarizations are
orthogonal each other. Hereinafter, the respective lights of the polarized
waves separated in the PBS51 are called "X polarization light" and "Y
polarization light".
The X polarization light and the Y polarization light are modulated
with the QPSK modulation by data 55 and 56 which are outputted from a
control circuit 57 and applied to the electrode parts 58 and 59, respectively.
Incidentally, the intensity difference between the X polarization
light and the Y polarization light outputted from the optical transmitter 1
can be made small by setting the polarization direction of the light
outputted from the polarizer 4 so that the strength of the X polarization
light and the Y polarization light which are outputted from the PBS51 may
become equal. The polarization loss difference (polarization dependent
loss) that occurs between the X polarization light and the Y polarization
light can be made small by making the intensity difference of the X
polarization light and the Y polarization light small, so that the
transmission quality difference can be reduced.
The X polarization light and the Y polarization light are modulated
with the QPSK modulation as an I (Inphase) signal and a Q (Quadrature)
signal respectively. In Fig. 1, the light signal modulated as I signal is
indicated by X-I and Y-I, and the light signal modulated as a Q signal is
indicated by X-Q and Y-Q.
The X-I signal and the X-Q signal are modulated with the QPSK
modulation by data 55. The Y-I signal and the Y-Q signal are modulated
with the QPSK modulation by data 56. Further, a bias voltage for driving
the electrode part 58 and the electrode part 59 is superposed by the control
circuit 57 on data 55 and data 56.
The X polarization light and the Y polarization light to which the
QPSK modulation has been performed are transmitted as the DP-QPSK
modulation signals after performed the polarization-combination in the
PBC (Polarization Beam Combiner) 52.
Figs. 2 to 5 are the figures which show a constellation of the
modulation signal in each part in the DP-QPSK modulator 5.
Fig. 2 indicates the constellation just after the modulation of the XI
signal modulated with PSK modulation on the I axis (Point A of Fig. 1).
Fig. 3 indicates the constellation just after the modulation of the X-Q
signal modulated with PSK modulation on the Q-axis (Point B of Fig. 1).
The X-Q signal has the phase rotation of nI2 to the X-I signal by the phase
shifter 53. The signals with the constellation shown in Fig. 2 and Fig. 3
have the phase rotation of x14 and are merged at the point C respectively.
Fig. 4 indicates the constellation of the signal to which the X-I signal and
X-Q signal are merged (Point C of Fig. 1). As a result, the modulation
signal of the X polarization light in the point C has the constellation as
shown in Fig. 4.
The constellations shown in Figs. 2 to 4 are similar to that of the Y
polarization light in Fig. 1. As a result, the constellation of the light for
which the polarization-combination has been performed with the PBC52
becomes to be as shown in Fig. 5.
Supposing the bit rate of the PSK modulation indicated by the
constellations of Fig. 2 and Fig. 3 are 25Gbps respectively, the bit rate of
the QPSK signal indicated by the constellation of Fig. 4 is SOGbps, and the
optical transmitter 1 realizes the bit rates of lOOGbps for the polarizationcombination
of the X polarization light and the Y polarization light.
Incidentally, because the basic configuration and the operation of
the DP-QPSK modulator are known well, the more detailed description is
omitted. Further, the first exemplary embodiment has described the DPQPSK
modulator 5, in which the whole is configured by an optical
waveguide. However, the DP-QPSK modulator 5 may be configured by
connecting a plurality of optical parts with an optical fiber so that the
similar function may be realized.
By arranging the polarizer 4 in an input part of the DP-QPSK
modulator 5, the optical transmitter 1 of the first exemplary embodiment
described above can suppress the degradation of the polarization extinction
ratio of the RZ pulse inputted to the DP-QPSK modulator 5. As a result,
the optical transmitter 1 can output the DP-QPSK signal using RZ code
with high extinction ratio. That is, in an optical transmission system
using an RZ code, the optical transmitter 1 of the first exemplary
embodiment offers the effect that improves the quality of the transmission
signal.
As the modification of the first exemplary embodiment, an RZ
optical modulator which has only an RZ carver, a polarization regulator,
and an optical modulator can be considered.
In the RZ optical modulator having such configuration, the RZ
carver generates an RZ pulse train. The polarization regulator outputs the
light outputted from the RZ carver as linear polarization. Further, the
optical modulator modulates and outputs the RZ pulse train outputted from
the polarization regulator. Incidentally, in the modification of the first
exemplary embodiment, the optical modulator is not limited to a DP-QPSK
modulator. Further, as the polarization regulator, a polarizer may be used.
In the RZ optical modulator of the modification of such first
exemplary embodiment, an RZ pulse train with the high extinction ratio is
inputted to the optical modulator by using a polarization regulator. As a
result, the optical modulator can output a modulation signal with high
extinction ratio. Accordingly, in the optical transmission system using an
RZ code, the RZ optical modulator of the modification of the first
exemplary embodiment also offers the effect that the quality of the
transmission signal is improved.
[The second exemplary embodiment]
Fig. 6 is a figure which shows the configuration of the optical
modulator of the second exemplary embodiment of the present invention.
The optical transmitter 10 shown in Fig. 6 has a variable polarizer 41
instead of the polarizer 4 in comparison with the optical transmitter 1
shown in Fig. 1. And the CW light source 2, the RZ carver 3, the variable
polarizer 41 and DP-QPSK modulator 6 are connected each other with
polarization maintaining fibers.
By a control signal from outside, the variable polarizer 41 rotates
the polarization plane of the linearly polarized light outputted from the
variable polarizer 41. For example, a 112 wavelength plate can be
employed as the variable polarizer 41. The polarization plane of the
linearly polarized light can be rotated by rotating the 112 wavelength plate
which makes a linearly polarized light pass through by a control signal.
Otherwise, the variable polarizer 41 may have the configuration in which
the linearly polarized light in turn passes through two pieces of the 114
wavelength plates connected in series. In the configuration using the 114
wavelength plate, the 114 wavelength plate in an input stde converts a
linearly polarized light into a circular polarization light and the 114 plate in
an output side outputs the circular polarization light as a linearly polarized
light. And the polarization plane of the linearly polarized light outputted
from the 114 wavelength plate in the output side can be rotated by rotating
the 114 wavelength plate in the output side by a control signal. Because
the functions of the CW light source 2 and the RZ carver 3 are similar to
the first embodiment figure, the description will be omitted.
The optical transmitter 10 is equipped with a PD (photo diode) 61
and a PD 62 for monitoring inside the DP-QPSK modulator 6. The PDs
61 and 62 for monitoring receive the lights respectively which are the
branching part of the X polarization light and the Y polarization light after
the modulation, and output monitor currents proportional to the strength of
the received lights.
Generally, in a QPSK modulator, the phase difference of an I signal
and a Q signal needs to be maintained in n12. For this reason, the ABC
(automatic bias control) control which monitors the light after the
modulation and feed-backs the result of the monitoring to the bias voltage
applied to the electrode part of the optical modulator is performed. By
the ABC control, a phase difference of modulated I signal and Q signal are
maintained in n12. Because the ABC control of a QPSK modulator is well
known and the ABC control does not relate directly with the configuration
of the present invention, the detailed description will be omitted. The
operation of the components of the DP-QPSK modulator 6 other than PDs
61, 62 and the control circuit 63 are similar to the DP-QPSK modulator 5
of the first exemplary embodiment.
An optical transmitter 10 improves the polarization extinction ratio
of an RZ pulse train by arranging the variable polarizer 41 between the RZ
carver 3 and the DP-QPSK modulator 6. As a result, the optical
transmitter 10 can output the DP-QPSK signal using an RZ code of high
extinction ratio like the optical transmitter of the first exemplary
embodiment.
Here, when the polarization extinction ratio of the light inputted to
the DP-QPSK modulator is low, the SNR of the output currents of the PDs
61 and 62 for monitoring which monitor the light intensity degrades and it
may not be possible to perform the high precision ABC control to the
respective QPSK modulators of both of the X and Y polarizations.
However, because the SNR deterioration of the monitor current by the
degradation of polarization extinction ratio can be suppressed by having
the variable polarizer 41, the optical transmitter 10 also has the effect that
the precision of the ABC control can be increased.
Moreover, the variable polarizer 41 can rotate the polarization
plane of the output of the RZ carver based on the monitor current outputted
by the PD 61 and 62 for monitoring. And the optical transmitter 10 can
vary the strength of the X polarization light and the Y polarization light
branched in the PBS 51 by rotating the polarization plane of the light
which the variable polarizer 41 outputs.
As stated in the first exemplary embodiment, when the polarization
loss difference (polarization dependent loss) occurs in the strength of the X
polarization light and the strength of the Y polarization light, the
difference may occur in the transmission quality between the signal of the
X polarization light and the signal of the Y polarization light to which the
polarization-combination has been performed. Accordingly, by rotating
the polarization plane of a variable polarizer 41 so that the monitor current
of the PD 61 for monitoring and the monitor current of the PD 62 for
monitoring may be equal, the strength of the X polarization light and the
strength of the Y polarization light after the modulation can be controlled
5 so as to become equal. Even when the internal loss of the DP-QPSK
modulator varies by a fluctuation of the environmental condition or the like,
as a result, the optical transmitter 10 can be controlled so that the strength
of the X polarization light and the strength of the Y polarization light
which are inputted to the PBC 52 may be accorded.
10 Thus, in an optical transmission system using an RZ code, the
optical transmitter 10 of the second exemplary embodiment also offers the
effect that the quality of the transmission signal is improved.
[The third exemplary embodiment]
Fig. 7 is a figure which shows a configuration of an optical
15 transmitter 20 of the third exemplary embodiment of the present invention.
The optical transmitter 20 shown in Fig. 7 includes a CW light source 2, an
RZ carver 3, a polarizer 4 and a DP-QPSK modulator 7. The CW light
source 2, the RZ carver 3, the polarizer 4 and the DP-QPSK modulator 7
are connected each other by polarization maintaining fibers.
2 0 Because the configuration and the operation of the CW light source
2, the RZ carver 3 and the polarizer 4 in the optical transmitter 20 are
similar to the optical transmitter 1 of the first exemplary embodiment, the
respective descriptions will be omitted.
The DP-QPSK modulator 7 has the different point that a Y branch
25 71 and a polarization rotating appliance 72 are included compared with the
DP-QPSK modulator 5 of the first exemplary embodiment. The Y branch
71 branches an inputted RZ pulse train in two optical paths simply. The
DP-QPSK modulator 7 branches the light in Y branch 71 and rotates the
polarization of the X polarization light by .n/2 so as to be orthogonal to the
polarization of the Y polarization light by using the polarization rotating
appliance 72 just before performing the polarization-combination in the
PBC 52. Here, as the polarization rotating appliance 72, for example, an
112 wavelength plate may be inserted on the optical path of the X
5 polarization light with the angle rotating the X polarization light by ~ 1 2 .
And the PBC 52 performs the polarization-combination of the X
polarization light and the Y polarization light, and generates the DP-QPSK
modulation signal. In the DP-QPSK modulator 7, the operation of the
components other than the Y branch 71 and the polarization rotating
10 appliance 72 is similar to the DP-QPSK modulator 5 of the first exemplary
embodiment.
The RZ pulse with high extinction ratio is inputted by the polarizer
4 to the DP-QPSK modulator 7 in the optical transmitter 20 equipped with
the DP-QPSK modulator 7 with such configuration. The DP-QPSK
15 modulator 7 performs the QPSK modulation to the signals branched into
two respectively using RZ pulses with high polarization extinction ratio.
And the DP-QPSK modulator 7 rotates the polarization of the X
polarization light modulated with QPSK and multiplexes the X polarization
light with the Y polarization light in the PBC 52. As a result, the optical
20 transmitter 20 can output the DP-QPSK signal using the RZ code with high
extinction ratio.
Thus, in an optical transmission system using an RZ code, the
optical transmitter 20 of the third exemplary embodiment also offers the
effect that the quality of the transmission signal is improved.
25 Incidentally, in the third exemplary embodiment, the configuration
of the DP-QPSK modulator 7 is not limited to an optical waveguide. The
DP-QPSK modulator 7 may be configured with connecting a plurality of
the optical parts by optical fibers so that the similar function may be
realized. The realization means of the polarization rotating appliance 72
is not limited to the 112 wavelength plates, and a polarization rotating
appliance using an optical waveguide, for example, may be used.
In the DP-QPSK modulator 7, the polarization plane rotates by n/2
before and behind the polarization rotating appliance 72. Since the
characteristic of the optical waveguide changes greatly depending on a
polarization, the predetermined performance of the optical modulator may
not be obtained when the polarization plane of the light signal is rotated by
the polarization rotating appliance 72 on the way of the optical waveguide.
For this reason, for example, the electrode parts 58 and 59 and the Y
branch 71 are made with an optical waveguide and the polarization rotating
appliance 72 and the PBC 52 may be used as an discrete parts. And the
DP-QPSK modulator 7 may be configured by connecting the polarization
rotating appliance 72, the PBC 52 and the electrode parts 58 and 59 by
polarization maintaining fibers.
Although it has been supposed that the modulation scheme of the
optical modulator is QPSK in the first to the third exemplary embodiments,
the modulation method is not limited to QPSK. For example, the
modulation method may be the QAM (quadrature amplitude modulation).
In this way, the exemplary embodiments of the present invention
have been described with reference to the first to the third exemplary
embodiments. However, the form in which the present invention is
applicable is not limited to the exemplary embodiments mentioned above.
Various changes which a person skilled in the art can understand in the
scope of the present invention can be applied to the configuration and the
detail description of the present invention.
This application is based upon and claims the benefit of priority
from Japanese Patent Application No. 20 11 - 147562 applied on July 1 st,
2011, the disclosure of which is incorporated herein in its entirety by
reference.
Reference Signs List
1, 10 and 20 Optical transmitter
2 CW light source
3 RZ carver
4 Polarizer
5, 6 and 7 DP-QPSK modulator
4 1 Variable polarizer
51 PBS
52 PBC
53 Phase shifter
55, 56 Data
58, 59, Electrode part
61, 62 PD
57, 63 Control circuit
71 Y branch
72 Phase rotator

[Claim 11
Claims
An RZ optical modulator comprising:
an RZ carver for outputting an optical pulse train of an RZ (return
5 to zero) code;
a polarization adjustment means for improving a polarization
extinction ratio of the optical pulse train outputted from the RZ carver and
outputting the optical pulse train whose extinction ratio is improved; and
an optical modulation means for modulating and outputting the
10 optical pulse train outputted from the polarization adjustment means.
[Claim 21
The RZ optical modulator according to claim 1 , wherein the optical
modulation means polarization-multiplexes the modulated optical pulse
15 train and outputs the polarization-multiplexed optical signal.
[Claim 31
The RZ optical modulator according to claim 2, further comprising:
a monitor means for outputting a light intensity of each of
20 polarizations of the modulated optical pulse train; and
a control means for controlling a polarization angle of the optical
pulse train to the optical modulation means outputted from the polarization
adjustment means based on an output of the monitor means.
25 [Claim 41
The RZ optical modulator according to any one of claims 1 to 3,
wherein the optical modulation means is a QPSK (differential quadrature
phase shift keying) modulator or a QAM (quadrature amplitude
I modulation) modulator.
[Claim 51
The RZ optical modulator according to any one of claims 1 to 4,
wherein the polarization adjustment means is a polarizer.
5
[Claim 61
An RZ optical modulation method comprising:
outputting an optical pulse train of RZ code;
improving a polarization extinction ratio of the optical pulse train ;
10 and
modulating the optical pulse train whose polarization extinction
ratio is improved and outputting the modulated optical pulse train.
[Claim 71
15 The RZ optical modulation method according to claim 6, wherein a
Dated this 3oth day of December 2013
Of Anand and Anand Advocates
Agents for the Applicants