U.S. patent application number 13/038895 was filed with the patent office on 2011-10-06 for mach-zehnder interferometer type optical modulator.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Jun-ichi HASHIMOTO.
Application Number | 20110243491 13/038895 |
Document ID | / |
Family ID | 44709776 |
Filed Date | 2011-10-06 |
United States Patent
Application |
20110243491 |
Kind Code |
A1 |
HASHIMOTO; Jun-ichi |
October 6, 2011 |
MACH-ZEHNDER INTERFEROMETER TYPE OPTICAL MODULATOR
Abstract
A Mach-Zehnder interferometer type optical modulator includes a
first end facet and a reflecting portion opposing the first end
facet; a single optical coupler including input and output ports,
the optical coupler being disposed between the first end facet and
the reflecting portion; first and second optical waveguides that
are connected to the input ports of the optical coupler; third and
fourth optical waveguides that are connected to the output ports of
the optical coupler; and a phase shifting section disposed between
the optical coupler and the reflecting portion. The phase shifting
section includes a first optical waveguide structure constituting
part of the third optical waveguide; a first upper electrode on the
first optical waveguide structure; a second optical waveguide
structure constituting part of the fourth optical waveguide; and a
second upper electrode on the second optical waveguide
structure.
Inventors: |
HASHIMOTO; Jun-ichi;
(Yokohama-shi, JP) |
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
Osaka
JP
|
Family ID: |
44709776 |
Appl. No.: |
13/038895 |
Filed: |
March 2, 2011 |
Current U.S.
Class: |
385/3 |
Current CPC
Class: |
G02F 2201/34 20130101;
G02F 1/035 20130101; G02F 2201/17 20130101; G02F 1/212 20210101;
G02F 1/225 20130101 |
Class at
Publication: |
385/3 |
International
Class: |
G02F 1/035 20060101
G02F001/035 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 1, 2010 |
JP |
2010-085243 |
Claims
1. A Mach-Zehnder interferometer type optical modulator comprising:
a first end facet and a reflecting portion opposing the first end
facet; a single optical coupler including input ports and output
ports, the optical coupler being disposed between the first end
facet and the reflecting portion; a first optical waveguide and a
second optical waveguide that extend from the first end facet and
are connected to the input ports of the optical coupler; a third
optical waveguide and a fourth optical waveguide that extend from
the reflecting portion and are connected to the output ports of the
optical coupler; and a phase shifting section that controls a phase
of light propagating in at least one of the third and fourth
optical waveguides, the phase shifting section being disposed
between the optical coupler and the reflecting portion, wherein the
phase shifting section includes: a first optical waveguide
structure constituting part of the third optical waveguide; a first
upper electrode on the first optical waveguide structure; a second
optical waveguide structure constituting part of the fourth optical
waveguide; and a second upper electrode on the second optical
waveguide structure.
2. The Mach-Zehnder interferometer type optical modulator according
to claim 1, wherein the first optical waveguide structure in the
phase shifting section includes a first lower cladding layer, a
first core layer and a first upper cladding layer, the second
optical waveguide structure in the phase shifting section includes
a second lower cladding layer, a second core layer and a second
upper cladding layer, the first upper electrode is disposed on the
first upper cladding layer of the first optical waveguide
structure, and the second upper electrode is disposed on the second
upper cladding layer of the second optical waveguide structure.
3. The Mach-Zehnder interferometer type optical modulator according
to claim 1, wherein the optical coupler is a multimode interference
coupler including two input ports and two output ports which are a
through port and a cross port, respectively, and the optical
coupler is configured so that a phase of guided light propagating
from one of the input ports to the cross port is delayed by .pi./2
with respect to a phase of guided light propagating from the input
port to the through port.
4. The Mach-Zehnder interferometer type optical modulator according
to claim 1, further comprising: a phase adjusting section that
adjusts a phase of light propagating in at least one of the third
and fourth optical waveguides independently from the phase shifting
section, the phase adjusting section being disposed between the
phase shifting section and the optical coupler, wherein the phase
adjusting section includes: a third optical waveguide structure
constituting part of the third optical waveguide; a third upper
electrode disposed on the third optical waveguide structure; a
fourth optical waveguide structure constituting part of the fourth
optical waveguide; and a fourth upper electrode disposed on the
fourth optical waveguide structure.
5. The Mach-Zehnder interferometer type optical modulator according
to claim 1, wherein the reflecting portion includes a plurality of
distributed Bragg reflectors; the distributed Bragg reflectors
include a fifth optical waveguide structure and a sixth optical
waveguide structure that constitute part of the third and fourth
optical waveguides, respectively, the fifth and sixth optical
waveguide structures each including a core layer in which a
diffraction grating having periodic projections and recesses is
formed.
6. The Mach-Zehnder interferometer type optical modulator according
to claim 5, further comprising: a plurality of monitoring portions
respectively connected to the distributed Bragg reflectors, wherein
the monitoring portions include photo-receiving elements that
convert light output from the distributed Bragg reflectors into
electrical signals.
7. The Mach-Zehnder interferometer type optical modulator according
to claim 1, wherein the reflecting portion includes a plurality of
distributed Bragg reflectors optically connected to the third and
fourth optical waveguides, and the distributed Bragg reflectors
each include a diffraction grating structure including a plurality
of semiconductor portions and a plurality of dielectric portions
periodically and alternately arranged in a waveguiding
direction.
8. The Mach-Zehnder interferometer type optical modulator according
to claim 7, wherein the plurality of semiconductor portions of the
distributed Bragg reflectors are integrally formed across the
distributed Bragg reflectors.
9. The Mach-Zehnder interferometer type optical modulator according
to claim 7, wherein the plurality of dielectric portions of the
distributed Bragg reflectors are composed of benzocyclobutene resin
or polyimide resin.
10. The Mach-Zehnder interferometer type optical modulator
according to claim 1, wherein the reflecting portion includes a
plurality of distributed Bragg reflectors optically connected to
the third and fourth optical waveguides, and the distributed Bragg
reflectors each have a diffraction grating structure including a
plurality of semiconductor portions and a plurality of air gaps
that are alternately and periodically arranged in a waveguiding
direction.
11. The Mach-Zehnder interferometer type optical modulator
according to claim 1, wherein the reflecting portion is a second
end facet that opposes the first end facet and has a
high-reflection film formed thereon.
12. The Mach-Zehnder interferometer type optical modulator
according to claim 11, wherein the phase shifting section is
disposed apart from the second end facet by a predetermined
gap.
13. The Mach-Zehnder interferometer type optical modulator
according to claim 1, wherein the first end facet includes an
anti-reflection film formed thereon.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to Mach-Zehnder interferometer
type optical modulators.
[0003] 2. Description of the Related Art
[0004] Patent Document 1 (Japanese Unexamined Patent Application
Publication No. 62-183406) describes a waveguide-type optical
interferometer. This waveguide-type optical interferometer includes
a substrate, two optical guides composed of glass or a plastic
formed on the substrate, two optical couplers that connect the
optical waveguides to each other at different positions, and phase
shifters disposed in the optical waveguides between the optical
couplers. Each phase shifter includes a heater disposed on the
optical waveguide. The optical path length of the optical waveguide
is changed by controlling the temperature of the optical waveguide
by heating the heater of the phase shifter.
[0005] In recent years, optical modulators that modulate light in
response to electric signals from outside have become one of the
essential components in configuring optical fiber communication
systems and optical information processing systems. In particular,
a Mach-Zehnder interferometer type optical modulator that uses a
waveguide-type optical interferometer described in Patent Document
1 enables high-speed modulation of 40 Gbps or higher. Since
Mach-Zehnder interferometer type optical modulators have a low
wavelength chirp under high-speed modulation, Mach-Zehnder
interferometer type optical modulators can be used for future ultra
high-speed, high-capacity optical communication systems. In
particular, Mach-Zehnder interferometer type optical modulators
composed of semiconductors are small in size, have low power
consumption, and can be monolithically integrated with other
semiconductor optical devices such as a laser diode through to
achieve wider versatility.
[0006] A Mach-Zehnder interferometer type optical modulator has an
optical waveguide structure constituted by an upper cladding layer,
a lower cladding layer, and a core layer between these cladding
layers. The core layer is composed of a material having a
refractive index higher than those of the upper cladding layer and
the lower cladding layer. In particular, an optical waveguide
structure of a semiconductor optical device has a pin structure in
which one of the upper and lower cladding layers is composed of an
n-type semiconductor, the other of the upper and lower cladding
layers is composed of a p-type semiconductor, and the core layer is
composed of an undoped semiconductor.
[0007] FIGS. 17, 18A and 18B show a Mach-Zehnder interferometer
type optical modulator 100 having a pin structure. FIG. 17 is a
plan view of the Mach-Zehnder interferometer type optical modulator
100. FIG. 18A is a cross-sectional view taken along line
XVIIIa-XVIIIa in FIG. 17. FIG. 18B is a cross-sectional view taken
along line XVIIIb-XVIIIb in FIG. 17.
[0008] As shown in FIG. 17, the Mach-Zehnder interferometer type
optical modulator 100 includes a phase shifting section 110, an
input optical coupler 120, an output optical coupler 130, and six
optical waveguides 140a, 140b, 140c, 150a, 150b, and 150c. These
components are formed on an n-type semiconductor substrate 101
(refer to FIGS. 18A and 18B). Each of the waveguides 140b and 150b
has one end connected to the input optical coupler 120 and the
other end connected to the output optical coupler 130. The phase
shifting section 110 is interposed between the input optical
coupler 120 and the output optical coupler 130. Upper electrodes
111a and 111b are respectively disposed on the optical waveguides
140b and 150b in the phase shifting section 110.
[0009] The optical waveguides 140a and 150a each extend from one
end 100a of the Mach-Zehnder interferometer type optical modulator
100 to the input optical coupler 120. The optical waveguides 140c
and 150c each extend from output optical coupler 130 to the other
end 100b of the Mach-Zehnder interferometer type optical modulator
100.
[0010] Referring now to FIGS. 18A and 18B, the Mach-Zehnder
interferometer type optical modulator 100 includes an n-type lower
cladding layer 103, core layers 104a and 104b, p-type upper
cladding layers 105a and 105b, and p-type contact layers 106a and
106b. The core layer 104a is interposed between the n-type lower
cladding layer 103 and the p-type upper cladding layer 105a. The
core layer 104b is interposed between the n-type lower cladding
layer 103 and the p-type upper cladding layer 105b. The p-type
contact layers 106a and 106b are disposed on the p-type upper
cladding layers 105a and 105b, respectively. The upper electrodes
111a and 111b are disposed on the p-type contact layers 106a and
106b, respectively. A cathode electrode 112 is formed on the back
of the n-type semiconductor substrate 101.
[0011] A part of the n-type lower cladding layer 103, the core
layer 104a, the p-type upper cladding layer 105a, and the p-type
contact layer 106a form a mesa structure 107a. The mesa structure
107a constitutes the optical waveguide 140b. Similarly, another
part of the n-type lower cladding layer 103, the core layer 104b,
the p-type upper cladding layer 105b, and the p-type contact layer
106b form another mesa structure 107b. The mesa structure 107b
constitutes the optical waveguide 150b. Side surfaces of the mesa
structures 107a and 107b are buried by, for example, a polyimide
resin 108.
[0012] According to the Mach-Zehnder interferometer type optical
modulator 100, the refractive indices of the core layers 104a and
104b can be changed by applying a reverse bias voltage between the
cathode electrode 112 and the upper electrodes 111a and 111b. As a
result, the phase of the light guided in the core layers 104a and
104b can be shifted.
SUMMARY OF THE INVENTION
[0013] As shown in FIG. 17, the device length L in the waveguiding
direction of the Mach-Zehnder interferometer type optical modulator
100 is the length from the end 100a to the end 100b of the
Mach-Zehnder interferometer type optical modulator 100. The device
length L in the waveguiding direction is relatively large, e.g., 3
mm to 5 mm. Therefore, it is difficult to install the Mach-Zehnder
interferometer type optical modulator 100 in a small-size optical
module.
[0014] A first aspect of the present invention provides a
Mach-Zehnder interferometer type optical modulator including a
first end facet and a reflecting portion opposing the first end
facet; a single optical coupler including input ports and output
ports, the optical coupler being disposed between the first end
facet and the reflecting portion; a first optical waveguide and a
second optical waveguide that extend from the first end facet and
are connected to the input ports of the optical coupler; a third
optical waveguide and a fourth optical waveguide that extend from
the reflecting portion and are connected to the output ports of the
optical coupler; and a phase shifting section that controls a phase
of light propagating in at least one of the third and fourth
optical waveguides, the phase shifting section being disposed
between the optical coupler and the reflecting portion. The phase
shifting section includes a first optical waveguide structure
constituting part of the third optical waveguide; a first upper
electrode on the first optical waveguide structure; a second
optical waveguide structure constituting part of the fourth optical
waveguide; and a second upper electrode on the second optical
waveguide structure.
[0015] According to this Mach-Zehnder interferometer type optical
modulator, the first and second optical waveguides extend from the
first end facet and are connected to the input ports of the optical
coupler. Meanwhile, the third and fourth optical waveguides that
extend from the reflecting portion and are connected to the output
ports of the optical coupler are also provided. Incoming light
enters from the first-end facet-side of the first or second optical
waveguide, propagates through the optical coupler and the third and
fourth optical waveguides, and is reflected by the reflecting
portion so as to again propagate in the third and fourth optical
waveguides. The Mach-Zehnder interferometer type optical modulator
also includes the phase shifting section. The phase shifting
section includes the first and second optical waveguide structures
respectively constituting part of the third and fourth optical
waveguides. Since the first and second upper electrodes are
respectively disposed on the first and second optical waveguide
structures, the phase of light guided in at least one of the third
and fourth optical waveguides can be controlled by applying a
modulated signal (for example, modulated voltage) to the first or
second upper electrode. Lights propagating in the third and fourth
optical waveguides from the phase shifting section enter the output
ports of the optical coupler. Modulated light according to the
modulated signal can be output from the first-end facet-side of the
first or second optical waveguide through the optical coupler. A
conventional Mach-Zehnder interferometer type optical modulator
shown in FIG. 17, 18A or 18B has at least two optical couplers of
the input optical coupler 120 and the output optical coupler 130.
On the other hand, this Mach-Zehnder interferometer type optical
modulator has a single optical coupler. Thus, the Mach-Zehnder
interferometer type optical modulator can be operated by using only
the left half of the region of the conventional Mach-Zehnder
interferometer type optical modulator shown in FIG. 17, 18A or 18B.
Accordingly, the device length in the waveguiding direction can be
shortened. Thus, the Mach-Zehnder interferometer type optical
modulator can be easily mounted in a small-size optical module.
[0016] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the first optical
waveguide structure in the phase shifting section may include a
first lower cladding layer, a first core layer and a first upper
cladding layer. Furthermore, the second optical waveguide structure
in the phase shifting section includes a second lower cladding
layer, a second core layer and a second upper cladding layer. The
first upper electrode is disposed on the first upper cladding layer
of the first optical waveguide structure, and the second upper
electrode is disposed on the second upper cladding layer of the
second optical waveguide structure. Light propagating in the first
optical waveguide structure is confined in the first core layer
sandwiched between the first lower cladding layer and the first
upper cladding layer. Similarly, light propagating in the second
optical waveguide structure is confined in the second core layer
sandwiched between the second lower cladding layer and the second
upper cladding layer. A phase of guided light in the first or
second optical waveguide structures can be changed by changing a
refractive index of at least one of the first and second core
layers according to applying a modulated signal to the first or
second upper electrodes.
[0017] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the optical coupler
may be a multimode interference coupler including two input ports
and two output ports which are a through port and a cross port,
respectively. The optical coupler may be configured so that a phase
of guided light propagating from one of the input ports to the
cross port is delayed by .pi./2 with respect to a phase of guided
light propagating from the input port to the through port.
[0018] According to the Mach-Zehnder interferometer type optical
modulator, the phase difference is generated between light guided
in the third optical waveguide and light guided in the fourth
optical waveguide by applying a modulated signal to the upper
electrode of the phase shifting section. Moreover, the optical
coupler is configured so that a phase difference of .pi./2 is
generated between the light propagating from the input port of the
optical coupler to the through port, i.e., the output port, and the
light propagating from the in put port of the optical coupler to
the cross port. Both the phase difference between light guided in
the third optical waveguide and light guided in the fourth optical
waveguide in the phase shifting section and the phase difference
generated as a result of propagation in the optical coupler are
utilized. As a result, the continuous wave (CW) light input to one
of the input ports of the optical coupler is modulated by the
modulated signal and output from the other input port as a
modulated signal light.
[0019] The Mach-Zehnder interferometer type optical modulator may
further include a phase adjusting section that adjusts a phase of
light propagating in at least one of the third and fourth optical
waveguides independently from the phase shifting section, the phase
adjusting section being disposed between the phase shifting section
and the optical coupler. The phase adjusting section may include a
third optical waveguide structure constituting part of the third
optical waveguide; a third upper electrode disposed on the third
optical waveguide structure; a fourth optical waveguide structure
constituting part of the fourth optical waveguide; and a fourth
upper electrode disposed on the fourth optical waveguide
structure.
[0020] The guided light beams that have propagated through the
third and fourth optical waveguides and have been reflected by the
reflecting portion may have different reflective characteristics
due to the error that occurs during the device production. The
difference in phase shift amount occurs at the reflecting portion
due to such a difference in reflective characteristics. The
difference in phase shift amount differs from one device to
another. As a result, the optimum value of modulated signal (for
example, modulated voltage) applied to the phase shifting section
differs from one device to another, and it becomes difficult to
ensure stable operation and reproduction of the device
characteristics. To address this issue, the Mach-Zehnder
interferometer type optical modulator may further include a phase
adjusting section configured to adjust a phase of light by
adjusting the refractive index of at least one of the third and
fourth optical waveguides independently from the phase shifting
section. The difference in phase shift amount at the reflecting
portion can be compensated by adjusting a phase of guided light
through the phase adjusting section. Thus, the optimum value of the
modulated signal applying to the phase shifting section can be
easily maintained substantially constant. The stable operation and
reproduction of device characteristics can thus be easily
achieved.
[0021] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the reflecting
portion may include a plurality of distributed Bragg reflectors.
The distributed Bragg reflectors may include a fifth optical
waveguide structure and a sixth optical waveguide structure that
constitute part of the third and fourth optical waveguides,
respectively, the fifth and sixth optical waveguide structures each
including a core layer in which a diffraction grating having
periodic projections and recesses is formed.
[0022] The Mach-Zehnder interferometer type optical modulator may
further include a plurality of monitoring portions respectively
connected to the distributed Bragg reflectors. The monitoring
portions may include photo-receiving elements that convert light
output from the distributed Bragg reflectors into electrical
signals.
[0023] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the reflecting
portion may include a plurality of distributed Bragg reflectors
optically connected to the third and fourth optical waveguides. The
distributed Bragg reflectors may each include a diffraction grating
structure including a plurality of semiconductor portions and a
plurality of dielectric portions periodically and alternately
arranged in a waveguiding direction. Since the difference in
refractive index between semiconductors and dielectric materials is
generally great, the reflecting portion of the Mach-Zehnder
interferometer type optical modulator will have a higher
reflectivity than that of the reflection portion having distributed
Bragg reflectors composed of semiconductor materials.
[0024] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the plurality of
semiconductor portions of the distributed Bragg reflectors may be
integrally formed across the distributed Bragg reflectors.
Furthermore, the plurality of dielectric portions of the
distributed Bragg reflectors may be composed of benzocyclobutene
resin or polyimide resin.
[0025] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the reflecting
portion may include a plurality of distributed Bragg reflectors
optically connected to the third and fourth optical waveguides. The
distributed Bragg reflectors may each have a diffraction grating
structure including a plurality of semiconductor portions and a
plurality of air gaps that are alternately and periodically
arranged in a waveguiding direction. The difference in refractive
index between semiconductors and air is greater than that between
semiconductors and dielectric materials. Thus, according to the
Mach-Zehnder interferometer type optical modulator including such
distributed Bragg reflectors, the reflecting portion achieves a
high reflectivity.
[0026] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the reflecting
portion may be a second end facet that opposes the first end facet
and has a high-reflection film formed thereon.
[0027] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the phase shifting
section is disposed apart from the second end facet by a
predetermined gap. The second end facet is, for example, formed by
cleaving. When the second end facet is terminated with the phase
shifting section including the upper electrode, the electrode
material near the second end facet may adhere on the second end
facet during cleavage. However, according to this Mach-Zehnder
interferometer type optical modulator, the second end facet is
terminated with the waveguiding section not having an upper
electrode and no electrode material is present near the second end
facet. Therefore, adhesion of the electrode on the end facet can be
suppressed.
[0028] In the Mach-Zehnder interferometer type optical modulator
according to the present invention, preferably the first end facet
may include an anti-reflection film formed thereon.
[0029] According to the Mach-Zehnder interferometer type optical
modulator, since the anti-reflection film is formed on the first
end facet that transmits light, reflection of incoming light and
outgoing light by the first end facet can be suppressed. Moreover,
the anti-reflection film suppresses degradation of the optical
coupling efficiency between the Mach-Zehnder interferometer type
optical modulator and optical fibers connected to the Mach-Zehnder
interferometer type optical modulator. Since the anti-reflection
film is provided, outgoing light can be suppressed from being
reflected by the first end facet and returning to the interior of
the modulator. Accordingly, the adverse effects on the modulation
operation of the modulator can be diminished.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a plan view showing an example of a Mach-Zehnder
interferometer.
[0031] FIG. 2A is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator shown in FIG. 1 taken along
line IIA-IIA; and FIG. 2B is a cross-sectional view of the
Mach-Zehnder interferometer type optical modulator shown in FIG. 1
taken along line IIB-IIB.
[0032] FIG. 3 is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator shown in FIG. 1 taken along
line III-III.
[0033] FIG. 4 is a plan view showing Modification Example 1 of the
Mach-Zehnder interferometer type optical modulator.
[0034] FIG. 5 is a plan view showing Modification Example 2 of the
Mach-Zehnder interferometer type optical modulator.
[0035] FIG. 6 is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator shown in FIG. 5 taken along
line VI-VI.
[0036] FIG. 7 is a plan view showing Modification Example 3 of the
Mach-Zehnder interferometer type optical modulator.
[0037] FIG. 8 is a plan view showing Modification Example 4 of the
Mach-Zehnder interferometer type optical modulator.
[0038] FIG. 9 is a plan view showing Modification Example 5 of the
Mach-Zehnder interferometer type optical modulator.
[0039] FIG. 10 is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator shown in FIG. 9 taken along
line X-X.
[0040] FIG. 11 is a plan view showing Modification Example 6 of the
Mach-Zehnder interferometer type optical modulator.
[0041] FIG. 12 is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulators shown in FIGS. 11 and 13
taken along line XII-XII.
[0042] FIG. 13 is a plan view showing Modification Example 7 of a
Mach-Zehnder interferometer type optical modulator.
[0043] FIG. 14 is a plan view showing Modification Example 8 of a
Mach-Zehnder interferometer type optical modulator.
[0044] FIG. 15 is a plan view showing Modification Example 9 of a
Mach-Zehnder interferometer type optical modulator.
[0045] FIG. 16 is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator taken along line XVI-XVI.
[0046] FIG. 17 is a plan view showing one example of an existing
Mach-Zehnder interferometer type optical modulator.
[0047] FIG. 18A is a cross-sectional view taken along line
XVIIIa-XVIIIa in FIG. 17 and FIG. 18B is a cross-sectional view
taken along line XVIIIb-XVIIIb in FIG. 17.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] Embodiments of the Mach-Zehnder interferometer type optical
modulator of the present invention will now be described in detail
with reference to the attached drawings. In the description of the
drawings, the same components are given the same reference numerals
and the description therefor is omitted to avoid redundancy.
First Embodiment
[0049] FIG. 1 is a plan view showing an example of a Mach-Zehnder
interferometer type optical modulator. Referring to FIG. 1, a
Mach-Zehnder interferometer type optical modulator 1A of this
embodiment has a first end facet 1a that transmits light and a
second end facet 1b that reflects light. The second end facet 1b is
the reflecting portion of this embodiment. For example, a
high-reflection film HR is formed on the second end facet 1b.
[0050] The Mach-Zehnder interferometer type optical modulator 1A of
this embodiment includes a phase shifting section 10, waveguiding
sections 20A and 20B, and an optical coupler 30. The optical
coupler 30 is disposed between the waveguiding section 20A and the
waveguiding section 20B. The waveguiding section 20B is disposed
between the optical coupler 30 and the phase shifting section 10.
The phase shifting section 10, the waveguiding sections 20A and
20B, and the optical coupler 30 are formed on an n-type
semiconductor substrate 4 as shown in FIGS. 2A, 2B and 3. An n-type
InP substrate can be used as the n-type semiconductor substrate 4,
for example.
[0051] The Mach-Zehnder interferometer type optical modulator 1A
includes four optical waveguides 2A, 2B, 3A and 3B. According to
this embodiment, the optical waveguide 2A is a first optical
waveguide, the optical waveguide 3A is a second optical waveguide,
the optical waveguide 2B is a third optical waveguide and the
optical waveguide 3B is a fourth optical waveguide.
[0052] The optical waveguides 2A and 3A are formed on the
semiconductor substrate 4. The optical waveguides 2A and 3A extend
from the first end facet 1a to the optical coupler 30. The optical
waveguides 2A and 3A are disposed in the waveguiding section 20A.
The optical waveguides 2A and 3A are provided in parallel with each
other in an extending direction. The optical waveguides 2B and 3B
are formed on the semiconductor substrate 4. The optical waveguides
2B and 3B and extend from the optical coupler 30 to the second end
facet 1b. The optical waveguides 2B and 3B extend across the
waveguiding section 20B and the phase shifting section 10. The
optical waveguides 2B and 3B are provided in parallel with each
other in an extending direction. The optical waveguides 2A and 3A
may have the same optical length. However, the optical waveguides
2A and 3A may have different optical lengths. The optical
waveguides 2B and 3B may have the same optical length. However, the
optical waveguides 2B and 3B may have different optical
lengths.
[0053] The first end facet 1a and the second end facet 1b each
extend in a direction intersecting the waveguiding direction of the
optical waveguides 2A, 2B, 3A, and 3B, and are arranged to oppose
each other. A first upper electrode 11a and a second upper
electrode 11b are respectively disposed on the optical waveguides
2B and 3B in the phase shifting section 10.
[0054] Incoming light from outside is entered into one of the
optical waveguides 2A and 3A. In the example shown in FIG. 1, the
optical waveguide 2A is optically coupled to an external light
source S and incoming light L1 from outside is input to the optical
waveguide 2A. The light source S may be a distributed-feedback
(DFB) laser.
[0055] The optical coupler 30 couples or branches the guided light.
The optical coupler 30 branches the light, which has been input to
the Mach-Zehnder interferometer type optical modulator 1A from
outside through one of the optical waveguides 2A and 3A, to the
optical waveguides 2B and 3B. The optical coupler 30 may be
configured as a multimode interference (MMI) optical coupler having
two input ports and two output ports.
[0056] The optical coupler 30 has a first port P1 and a fourth port
P4 that function as input ports. The optical coupler 30 also has a
second port P2 and a third port P3 that function as output ports.
The second port P2 functions as a through port for the first port
P1. The third port P3 functions as a cross port for the first port
P1. The second port P2 functions as a cross port for the fourth
port P4. The third port P3 functions as a through port for the
fourth port P4.
[0057] One end of the optical waveguide 2A is connected to the
first end facet 1a of the Mach-Zehnder interferometer type optical
modulator 1A and the other end thereof is connected to the first
port P1 of the optical coupler 30. One end of the optical waveguide
3A is connected to the first end facet 1a of the Mach-Zehnder
interferometer type optical modulator 1A and the other end thereof
is connected to the fourth port P4 of the optical coupler 30. One
end of the optical waveguide 2B is connected to the second port P2
of the optical coupler 30 and the other end thereof is connected to
the second end facet 1b of the Mach-Zehnder interferometer type
optical modulator 1A. One end of the optical waveguide 3B is
connected to the third port P3 of the optical coupler 30 and the
other end thereof is connected to the second end facet 1b of the
Mach-Zehnder interferometer type optical modulator 1A.
[0058] In the direction from the first end facet 1a to the second
end facet 1b (the Y direction in the drawing), the optical coupler
30 branches the light input from one of the optical waveguides 2A
and 3A to the optical waveguides 2B and 3B. In the direction from
the second end facet 1b to the first end facet 1a, the optical
coupler 30 couples light guided through the optical waveguide 2B
and light guided through the optical waveguide 3B and branches the
coupled light to the optical waveguides 2A and 3A.
[0059] In the phase control section 10, the refractive indices of
the optical waveguides 2B and 3B are changed so that the phase of
light is controlled.
[0060] FIG. 2A is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator 1A shown in FIG. 1 taken
along line IIA-IIA. Referring to FIG. 2A, the phase shifting
section 10 constitutes part of the optical waveguides 2B and 3B.
The phase shifting section 10 includes a lower cladding layer 13
and two mesa structures 19a and 19b in addition to a lower
electrode 18 and the first and second upper electrodes 11a and 11b
described above. The lower cladding layer 13 covers the entirety of
a main surface 4a of the semiconductor substrate 4.
[0061] The mesa structure 19a is formed on the section
corresponding to the optical waveguide 2B on the lower cladding
layer 13. The mesa structure 19b is formed on the section
corresponding to the optical waveguide 3B on the lower cladding
layer 13.
[0062] The mesa structure 19a constitutes an optical waveguide
structure (first optical waveguide structure) of this embodiment.
The mesa structure 19a includes part of the lower cladding layer
13, a first core layer 14a, a first upper cladding layer 15a, and a
first contact layer 16a. The lower cladding layer 13 constitutes a
first lower cladding layer of this embodiment. The mesa structure
19b constitutes an optical waveguide structure (second optical
waveguide structure) of this embodiment. The mesa structure 19b
includes part of the lower cladding layer 13, a second core layer
14b, a second upper cladding layer 15b, and a second contact layer
16b. The lower cladding layer 13 also constitutes a second lower
cladding layer of this embodiment.
[0063] The lower cladding layer 13 is formed on the semiconductor
substrate 4. The first core layer 14a is formed on the lower
cladding layer 13. The first upper cladding layer 15a is formed on
the first core layer 14a. The refractive index of the first core
layer 14a is larger than those of the lower cladding layer 13 and
the first upper cladding layer 15a. The first core layer 14a may be
composed of an undoped semiconductor. These layers of the mesa
structure 19a constitute part of the optical waveguide 2B with the
first core layer 14a at the center. The lower cladding layer 13 may
be composed of, for example, an n-type semiconductor. The first
upper cladding layer 15a may be composed of, for example, a p-type
semiconductor.
[0064] The second core layer 14b is formed on the lower cladding
layer 13. The second upper cladding layer 15b is formed on the
second core layer 14b. The refractive index of the second core
layer 14b is larger than those of the lower cladding layer 13 and
the second upper cladding layer 15b. The second core layer 14b may
be composed of an undoped semiconductor. These layers of the mesa
structure 19b constitute part of the optical waveguide 3B with the
second core layer 14b at the center. The second upper cladding
layer 15b may be composed of, for example, a p-type
semiconductor.
[0065] The first and second core layers 14a, 14b may be composed of
a semiconductor such as GaInAsP, AlGaInAs, AlInAs, and GaInAs.
These semiconductor materials can be lattice-matched to InP.
Accordingly, the first and second core layers 14a, 14b having good
crystal quality can be formed on the InP substrate. These
semiconductor materials also have a band gap energy corresponding
to the wavelength band used in optical fiber communication, i.e.,
1.3 .mu.m to 1.55 .mu.m. Accordingly, when the first and second
core layers 14a, 14b are composed of such a semiconductor material,
an optical modulator suitable for optical communication usage can
be obtained. The first and second core layers 14a, 14b may be a
single layer (bulk layer) or may have a quantum well structure
constituted by alternately stacked well layers and barrier
layers.
[0066] The lower cladding layer 13 and the first and second upper
cladding layers 15a, 15b are preferably composed of a semiconductor
material having a refractive index smaller than that of the first
core layer 14a or the second core layer 14b. For example, the lower
cladding layer 13 and the first and second upper cladding layers
15a, 15b may be composed of a semiconductor material selected from
InP, GaInAsP, AlGaInAs, and AlInAs. In the structure having a lower
cladding layer 13 composed of InP and first and second upper
cladding layers 15a, 15b composed of InP, carriers and light
guiding in the optical waveguide can be well confined in the first
and second core layers 14a, 14b.
[0067] The first contact layer 16a is disposed on the first upper
cladding layer 15a. The second contact layer 16b is disposed on the
second upper cladding layer 15b. The contact layers 16a and 16b may
be composed of, GaInAs or GaInAsP, for example.
[0068] The first upper electrode 11a is formed on the first contact
layer 16a. The second upper electrode 11b is formed on the second
contact layer 16b. The lower electrode 18 is formed on a back
surface 4b of the semiconductor substrate 4.
[0069] Both side surfaces of the mesa structures 19a and 19b are
buried by a resin layer 17 composed of a benzocyclobutene (BCB)
resin or a polyimide resin.
[0070] FIG. 2B is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator 1A shown in FIG. 1 taken
along line IIB-IIB. Referring to FIG. 2B, the waveguiding sections
20A and 20B have a lower cladding layer 23 and a mesa structure 29.
The mesa structure 29 is an optical waveguide structure of this
embodiment. The mesa structure 29 includes part of the lower
cladding layer 23, a core layer 24, and an upper cladding layer 25.
The mesa structure 29 is formed on the sections of the lower
cladding layer 23 respectively corresponding to the optical
waveguides 2A and 2B. The optical waveguide 2A and 2B include the
lower cladding layer 23, the core layer 24, and the upper cladding
layer 25.
[0071] The cladding layer 23 may be integrally formed with the
lower cladding layer 13 of the phase shifting section 10. The core
layer 24 may be integrally formed with the first core layer 14a of
the phase shifting section 10. The upper cladding layer 25 may be
integrally formed with the first upper cladding layer 15a of the
phase shifting section 10.
[0072] Both side surfaces of the mesa structure 29 is buried by a
resin layer 17 as with the mesa structures 19a and 19b of the phase
shifting section 10.
[0073] The core layers 24 is formed on the lower cladding layer 23.
The upper cladding layers 25 is formed on the core layers 24. The
refractive index of the core layer 24 is larger than the refractive
index of the lower cladding layer 23 and the upper cladding layer
25. These layers of the mesa structures 29 constitute part of the
optical waveguides 2A and 2B with the core layer 24 at the center.
The semiconductor material constituting the core layer 24 and the
internal structure of the core layer 24 are the same as those of
the core layers 14a and 14b described above. The semiconductor
material constituting the lower cladding layer 23 and the upper
cladding layer 25 is the same as those of the lower cladding layer
13 and the upper cladding layers 15a and 15b described above.
[0074] FIG. 3 is a cross-sectional view of the Mach-Zehnder
interferometer type optical modulator 1A shown in FIG. 1 taken
along line III-III. Referring to FIG. 3, the optical coupler 30
includes a lower cladding layer 33 and a mesa structure 39. The
lower cladding layer 33 may be integrally formed with the lower
cladding layer 13 and the lower cladding layer 23. The mesa
structure 39 constitutes an optical waveguide structure of this
embodiment. The mesa structure 39 is formed on the lower cladding
layer 33. The mesa structure 39 includes part of the lower cladding
layer 33, a core layer 34, and an upper cladding layer 35. Both
side surfaces of the mesa structure 39 are buried by the resin
layer 17 as with the mesa structures 19a, 19b, and 29.
[0075] The core layer 34 is formed on the lower cladding layer 33.
The upper cladding layer 35 is formed on the core layer 34. The
refractive index of the core layer 34 is larger than the refractive
index of the lower cladding layer 33 and the upper cladding layer
35. The core layer 34 of the optical coupler 30 is optically
connected to the core layer 24 described above. The semiconductor
material constituting the core layer 34 and the internal structure
of the core layer 34 are the same as those of the core layers 14a,
14b, and 24 described above. The semiconductor material
constituting the lower cladding layer 33 and the upper cladding
layer 35 is the same as those of the lower cladding layer 13 and
the upper cladding layers 15a and 15b described above.
[0076] Next, operation of the Mach-Zehnder interferometer type
optical modulator 1A is described. For the purposes of description,
it is assumed that the electrical signal voltage V for modulation
is applied only to the first upper electrode 11a of the phase
shifting section 10 of the optical waveguide 2B. Continuous wave
(CW) light having single wavelength is input from the light source
S to one of the optical waveguides 2A and 3A of the Mach-Zehnder
interferometer type optical modulator 1A. Alternatively, the light
may be input to the Mach-Zehnder interferometer type optical
modulator 1A from the light source S via an optical isolator H. In
the example shown in FIG. 1, incoming light L1 is input to the
optical waveguide 2A from the light source S via the optical
isolator H. The guided light that has propagated in the optical
waveguide 2A is input to the first port P1 of the optical coupler
30. The guided light input to the first port P1 propagates in the
optical coupler 30 and is branched to the second port P2 and the
third port P3. During this process, the second port P2 functions as
a through port and the third port P3 functions as a cross port.
[0077] The optical coupler 30 is configured such that the guided
light input to the optical coupler 30 through the first port P1 is
branched at a 1:1 power ratio and that the phase of the guided
light output to the third port P3 functioning as a cross port is
delayed by .pi./2 with respect to the second port P2 functioning as
a through port. For example, when a MMI coupler is used, the
lengths of the MMI optical coupler in the X and Y directions, the
distance between the first port P1 and the fourth port P4, and the
distance between the second port P2 and the third port P3 may be
adequately adjusted to change the optical path length between the
input port and the through port and the optical path length between
the input port and the cross port. In addition, the branching ratio
of light output to the through port and the cross port from the
input port can be adjusted. Accordingly, when a MMI optical coupler
is used as the optical coupler 30, an optical coupler 30 that
satisfies the above-described conditions can be prepared.
[0078] The guided light branched to the second port P2 and the
third port P3 is guided to the second end facet 1b-side through the
waveguiding section 20B of the optical waveguides 2B and 3B and the
phase shifting section 10, and is reflected by the second end facet
1b. The reflected light travels in the opposite direction, i.e.,
toward the first end facet 1a, through the optical waveguides 2B
and 3B and returns to the second port P2 and the third port P3.
[0079] An electrical signal voltage modulated with a 0-level and
1-level is applied to the first upper electrode 11a of the phase
shifting section 10 of the optical waveguide 2B. When a 0-level
electrical signal voltage is applied to the upper electrode 11a of
the phase shifting section 10, the 0-level electrical signal
voltage is adjusted so that the phase of light guided in the
optical waveguide 3B is ahead of the phase of light guided in the
optical waveguide 2B by .pi.. When a 1-level electrical signal
voltage is applied to the phase shifting section 10 of the optical
waveguide 2B, the 1-level electrical signal voltage is adjusted to
eliminate the difference between the phase of the guided light in
the optical guide 3B and the phase of the guided light in the
optical guide 2B.
[0080] First, the case in which a 1-level electrical signal voltage
is applied to the first upper electrode 11a of the phase shifting
section 10 of the optical waveguide 2B is discussed. A phase
difference does not occur between the light guided in the optical
waveguide 2B and the light guided in the optical waveguide 3B while
two guided light beams are guided through the optical waveguides 2B
and 3B and reflected back at the second end facet 1b. Accordingly,
when two guided light beams reach the second port P2 and the third
port P3 after reflecting at the second end facet 1b, the phase
relationship between the guided light at the third port P3 and the
guided light at the second port P2 is maintained. That is, the
phase of the guided light returning to the third port P3 is delayed
by .pi./2 with respect to the phase of the guided light returning
to the second port P2.
[0081] The two guided light reflected by the second end facet 1b
and reaching the second port P2 and the third port P3 are optically
coupled by the optical coupler 30, and are output to the first port
P1 or the fourth port P4. In the optical coupler 30, the branching
ratio and the phase relationship between the through port and the
cross port do not change at the time for branching or coupling the
guided lights. In this embodiment, the phase of light output from
the cross port of the MMI optical coupler is delayed by .pi./2 from
the phase of light output from the through port. Accordingly, at
the first port P1, the phase of light from the third port P3 is
delayed by .pi./2 with respect to the light from the second port P2
through the optical coupler 30. Similarly, at the fourth port P4,
the phase of light from the second port P2 is delayed by .pi./2
from the light from the third port P3 through the optical coupler
30.
[0082] The optical output from the fourth port P4 will now be
described. When the guided lights reach the second port 2 and the
third port 3 of the optical coupler 30 after reflecting at the
second end facet 1b, the phase of the guided light at the third
port P3 is delayed by .pi./2 with respect to the guided light at
the second port P2. Subsequently, as the guided light travel
through the optical coupler 30, the phase of light from the second
port P2 is delayed by .pi./2 with respect to the light from the
third port P3. As a result, the phase difference between the guided
light from the second port P2 and the guided light from the third
port P3 is canceled. Accordingly, the two guided light beams are
optically coupled in the fourth port P4 while intensifying each
other. Therefore, a 1-level optical signal is output to the fourth
port P4.
[0083] The optical output from the first port P1 will now be
described. When the guided lights reach the second port 2 and the
third port 3 of the optical coupler 30 after reflecting at the
second end facet 1b, the phase of the guided light at the third
port P3 is delayed by .pi./2 with respect to the phase of the
guided light at the second port P2. Subsequently, as the guided
light travels through the optical coupler 30, the phase of light
from the third port P3 to the first port P1 is further delayed by
.pi./2 with respect to the phase of the light from the second port
P2 to the first port P1. This means that the phase of light from
the third port P3 to the first port P1 is delayed by .pi. with
respect to the phase of the light from the second port P2 to the
first port P1. Accordingly, the two guided light beams are coupled
with each other while canceling each other. Therefore, a 0-level
optical signal is output to the first port P1.
[0084] The case in which a 0-level electrical signal voltage is
applied to the first upper electrode 11a of the phase shifting
section 10 of the optical waveguide 2B is discussed next. As
described above, while two guided light beams respectively branched
to the second port P2 and the third port P3 are guided through the
optical waveguides 2B and 3B and reflected back at the second end
facet 1b, the phase of the light guided in the optical waveguide 3B
is advanced by .pi. with respect to the phase of the light guided
in the optical waveguide 2B. The phase of the guided light at the
third port P3 is delayed by .pi./2 with respect to the phase of the
guided light at the second port P2 after branching the incoming
light through the optical coupler 30. Subsequently, the guided
light propagates in the optical waveguides 2B and 3B, is reflected
by the second end facet 1b and returns to the second port P2 or the
third port P3. At this time, the amount of change (.pi.) in the
phase of the light guided in the optical waveguide 3B relative to
the phase of the light guided in the optical waveguide 2B is added.
As a result, the phase of the guided light from the third port P3
is advanced by .pi./2 with respect to the phase of the guided light
from the second port P2.
[0085] The two guided light beams reaching the second port P2 and
the third port P3 are re-coupled by the optical coupler 30 and
light is output from the first port P1 or the fourth port P4. At
this time, at the first port P1, the phase of light from the third
port P3 is delayed by .pi./2 with respect to the phase of the light
from the second port P2 through the optical coupler 30. Meanwhile,
at the fourth port P4, the phase of light from the second port P2
is delayed by .pi./2 with respect to the phase of the light from
the third port P3 through the optical coupler 30.
[0086] The optical output from the fourth port P4 will now be
described. When the guided lights reach the second port P2 and the
third port P3 of the optical coupler 30 after reflecting at the
second facet 1b, the phase of the guided light at the third port P3
is advanced by .pi./2 with respect to the phase of the guided light
at the second port P2. Subsequently, as the guided light travels
through the optical coupler 30, the phase of light from the second
port P2 to the fourth port P4 is delayed by .pi./2 with respect to
the phase of the light from the third port P3 to the fourth port
P4. As a result, the phase difference between the guided light from
the second port P2 to the fourth port P4 and the guided light from
the third port P3 to the fourth port P4 is .pi.. Accordingly, the
two guided light beams are coupled with each other while canceling
each other at the fourth port P4. Therefore, a 0-level optical
signal is output to the fourth port P4.
[0087] The optical output from the first port P1 will now be
described. When the guided lights reach the second port P2 and the
third port P3 of the optical coupler 30 after reflecting at the
second facet 1b, the phase of the guided light at the third port P3
is advanced by .pi./2 with respect to the phase of the guided light
at the second port P2. Subsequently, as the guided light travels
through the optical coupler 30, the phase of light from the third
port P3 to the first port P1 is delayed by .pi./2 with respect to
the phase of the light from the second port P2 to the first port
P1. As a result, the phase difference between the guided light from
the second port P2 to the first port P1 and the guided light from
the third port P3 to the first port P1 is canceled. Accordingly,
the two guided light beams are optically coupled in the first port
P1 while intensifying each other. Therefore, a 1-level optical
signal is output to the first port P1.
[0088] In summary, when a 0-level electrical signal voltage is
applied to the first upper electrode 11a of the phase shifting
section 10 of the optical waveguide 2B, a 1-level optical signal is
output from the first port P1 and a 0-level optical signal is
output from the fourth port P4. Conversely, when a 1-level
electrical signal voltage is applied to the first upper electrode
11a of the phase shifting section 10 of the optical waveguide 2B, a
0-level optical signal is output from the first port P1 and a
1-level optical signal is output from the fourth port P4. In other
words, a modulated signal light having a pattern reverse of the
electrical signal voltage (that is, reverse phase pattern) is
output from the first port P1. Meanwhile, a modulated signal light
having the same pattern as the electrical signal voltage (that is,
in-phase pattern) is output from the fourth port P4. For example,
optical communication can be performed by using the modulated
signal light from the fourth port P4 as the outgoing light L3.
[0089] The Mach-Zehnder interferometer type optical modulator 1a of
this embodiment is a reflective-type device in which guided light
is branched or coupled by the single optical coupler 30 and is
reflected by the second end facet 1b functioning as a reflecting
portion. Accordingly, the device length in the waveguiding
direction can be shortened. Thus, the Mach-Zehnder interferometer
type optical modulator can be easily installed in a small-size
optical module. Since the device size is reduced, the device
capacitance can be decreased. As a result, high speed operation
exceeding, for example, 10 Gbps can be achieved.
[0090] In the embodiment above, only the case in which an
electrical signal voltage V is applied only to the first upper
electrode 11a of the phase shifting section 10 of the optical
waveguide 2B is described as an example. However, the voltage
application is not limited to this. An electrical signal voltage
may be applied only to the second upper electrode 11b of the phase
shifting section 10 of the optical waveguide 3B. Electrical signal
voltages can be simultaneously applied to the optical waveguides 2B
and 3B so that modulation can be achieved by a push-pull operation.
In this embodiment, to change the refractive index of the optical
waveguide 2B or 3B in the phase shifting section 10, electrical
signal voltage is applied to the first upper electrode 11a or the
second upper electrode 11b. However, signal current may be also
injected into the optical waveguides 2B and 3B through the first
upper electrode 11a or the second upper electrode 11b to change the
refractive index of the optical waveguide 2B or 3B in the phase
shifting section 10.
[0091] The operation of the light source S may become unstable if
the optical signal (outgoing light L2 in FIG. 1) output from the
first port P1 returns to the light source S via the optical fiber
F1. In order to prevent outgoing light L2 from returning to the
light source S, as shown in FIG. 1, an optical isolator H is
preferably optically connected between the light source S and the
optical waveguide 2A.
[0092] As described above, a high-reflection film HR may be formed
in the second end facet 1b of the Mach-Zehnder interferometer type
optical modulator 1A of this embodiment. The high-reflection film
HR may be, for example, a multilayer film including many dielectric
films having a high refractive index and a low refractive index
alternately stacked. Specific examples of the high-reflection film
HR include multilayer films including low-refractive-index
dielectric films composed of silicon oxide, silicon nitride, or
aluminum oxide and high-refractive-index dielectric films composed
of titanium oxide, tantalum oxide, or zirconium oxide that are
alternately stacked. Alternatively, a multilayer film including
low-refractive-index dielectric films composed of silicon oxide,
silicon nitride, or aluminum oxide and amorphous Si films which are
high-refractive index-semiconductor films that are alternately
stacked may be used as the high-reflection film HR. A metal film,
such a gold film, having a high reflectance may be used as the
high-reflection film HR. These high-reflection films can be easily
fabricated by using a vapor deposition machine, a sputtering
machine, or a chemical vapor deposition (CVD) machine. The
high-reflection film HR preferably has a reflectivity of 70% or
higher. And more preferably, the high-reflection film HR has a
reflectivity of 99% or higher.
[0093] As shown in FIG. 1, an anti-reflection film LR that prevents
reflection of the guided light may be provided in the first end
facet 1a of the Mach-Zehnder interferometer type optical modulator
1A. When the anti-reflection film LR is provided, the incoming
light L1 and the outgoing light L2 are prevented from being
reflected by the first end facet 1a. As a result, the optical
coupling efficiency between the optical fiber F1 and the
Mach-Zehnder interferometer type optical modulator 1A can be
improved. Moreover, the outgoing light L2 and L3 are prevented from
being reflected by the first end facet 1a and the adverse effects
on the modulation operation of the Mach-Zehnder interferometer type
optical modulator 1A can be diminished. A low-refractive-index film
such as a film composed of silicon oxide, silicon nitride, or
aluminum oxide can be used as the anti-reflection film LR. The
reflectivity of the anti-reflection film LR is preferably 1% or
less and more preferably less than 0.1%.
Modification Example 1
[0094] Modification Example 1 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 4 is a plan view showing Modification Example 1 of a
Mach-Zehnder interferometer type optical modulator.
[0095] A Mach-Zehnder interferometer type optical modulator 1B
shown in FIG. 4 differs from the Mach-Zehnder interferometer type
optical modulator 1A shown in FIG. 1 in that the Mach-Zehnder
interferometer type optical modulator 1B has a predetermined gap
between the phase shifting section 10 and the second end facet 1b.
The phase shifting section 10 is disposed apart from the second end
facet 1b by the predetermined gap. In other words, the Mach-Zehnder
interferometer type optical modulator 1B includes a waveguiding
section 20C between the phase shifting section 10 and the second
end facet 1b. The waveguiding section 20C extends from the phase
shifting section 10 to the second end facet 1b. The waveguiding
section 20C includes two optical waveguide structures constituting
part of the optical waveguides 2B and 3B, the optical waveguide
structures each including a lower cladding layer on the
semiconductor substrate 4, a core layer on the lower cladding
layer, and an upper cladding layer on the core layer. The optical
waveguide structures may be the same as one shown in FIG. 2B.
[0096] The second end facet 1b is formed by, for example, cleaving.
When the second end facet 1b is terminated with the phase shifting
section 10 having first and second upper electrodes 11a and 11b as
with the Mach-Zehnder interferometer type optical modulator 1A
shown in FIG. 1, the electrode material on the device surface may
adhere on the second end facet 1b during cleavage. When the
electrode material contacts the second end facet 1b, the leakage
current may increase at the end facet and crystal deterioration may
occur at the end facet in the phase shifting section 10.
[0097] In contrast, when the second end facet 1b is terminated with
the waveguiding section 20C not having electrodes shown in FIG. 4,
no electrode material is present near the second end facet 1b.
Thus, adhesion of the electrode onto the end facet mentioned above
does not occur. Thus, the increase in leakage current and crystal
deterioration at the end facet can be suppressed.
Modification Example 2
[0098] Modification Example 2 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 5 is a plan view showing Modification Example 2 of a
Mach-Zehnder interferometer type optical modulator. FIG. 6 is a
cross-sectional view of the Mach-Zehnder interferometer type
optical modulator shown in FIG. 5 taken along line VI-VI.
[0099] A Mach-Zehnder interferometer type optical modulator 1C
shown in FIGS. 5 and 6 differs from the Mach-Zehnder interferometer
type optical modulator 1A shown in FIG. 1 in that a semi-insulating
substrate is used as the semiconductor substrate. Accordingly, the
structure of a phase shifting section 110 of the Mach-Zehnder
interferometer type optical modulator 1C differs from the structure
of the phase shifting section 10 of the Mach-Zehnder interferometer
type optical modulator 1A.
[0100] According to Modification Example 2, an Fe-doped InP
substrate may be used as a semi-insulating semiconductor substrate
44. In this case, the lower electrodes 11c and 11d are formed on
the conductive lower cladding layer 13 on the semiconductor
substrate 44 in the phase shifting section 110. The lower cladding
layer 13 is composed of, for example, n-type InP.
[0101] According to the Mach-Zehnder interferometer type optical
modulator 1C of this modification example which uses an Fe-doped
InP substrate as the semiconductor substrate, the device
capacitance can be decreased compared to a device formed on an
electrically conductive semiconductor substrate such as an n-type
or a p-type InP substrate. Thus, high-speed modulation can be
realized.
Modification Example 3
[0102] Modification Example 3 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 7 is a plan view showing Modification Example 3 of a
Mach-Zehnder interferometer type optical modulator.
[0103] A Mach-Zehnder interferometer type optical modulator 1D
shown in FIG. 7 differs from the Mach-Zehnder interferometer type
optical modulator 1A shown in FIG. 1 in that the light source S is
integrally provided on the semiconductor substrate 4 which is part
of the modulator 1D. The light source S provides input guided light
to one of the optical waveguide 2A and the optical waveguide 3A.
Since the light source S is disposed on the semiconductor substrate
4 which is part of the Mach-Zehnder interferometer type optical
modulator 1D, the optical transmission system as a whole can
achieve size reduction.
[0104] When an optical isolator H is used, the optical isolator H
in addition to the light source S may be provided on the
semiconductor substrate 4 which is part of the Mach-Zehnder
interferometer type optical modulator 1D. The optical isolator H
which can be integrated on the same substrate as the light source S
may be an optical isolator in which a ferromagnetic metal composed
of iron or cobalt and a semiconductor are combined, for example. In
such a case, the optical transmission system as a whole including
the light source S and the optical isolator H can achieve size
reduction.
Modification Example 4
[0105] Modification Example 4 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 8 is a plan view showing Modification Example 4 of a
Mach-Zehnder interferometer type optical modulator.
[0106] A Mach-Zehnder interferometer type optical modulator 1E
shown in FIG. 8 differs from the Mach-Zehnder interferometer type
optical modulator 1A shown in FIG. 1 in that the modulator further
includes phase adjusting sections 50A and 50B. The phase adjusting
sections adjust the phase of the guided light independently from
the phase shifting section 10 by changing the refractive index of
at least one of the optical waveguide 2B and the optical waveguide
3B. The phase adjusting sections 50A and 50B are disposed between
the phase shifting section 10 and the optical coupler 30. The phase
adjusting sections 50A and 50B respectively constitute part of the
optical waveguide 2B and the optical waveguide 3B.
[0107] The phase adjusting sections 50A and 50B respectively
include optical waveguide structures (third and fourth optical
waveguide structures) each including a lower cladding layer on the
semiconductor substrate, a core layer on the lower cladding layer,
and an upper cladding layer on the core layer. Third and fourth
upper electrodes are disposed on the third and fourth optical
waveguide structures, respectively. In other words, the
cross-sectional structures of the phase adjusting sections 50A and
50B may be the same as that of the phase shifting section 10 shown
in FIG. 2A, for example.
[0108] The guided light beams that have propagated through the
optical waveguides 2B and 3B and have been reflected by the second
end facet 1b will have different reflective characteristics due to
the variations that occurs during production of the Mach-Zehnder
interferometer type optical modulators. Because of the difference
in reflective characteristic, a difference in phase shift amount is
generated at the second end facet 1b. The difference in phase shift
amount differs from one device to another. As a result, the optimum
value of the voltage applied to the first upper electrode 11a or
the second upper electrode 11b of the phase shifting section 10
differs among devices. Accordingly, the application voltage needs
to be adjusted for every device. The Mach-Zehnder interferometer
type optical modulator discussed here further includes, between the
phase shifting section 10 and the optical coupler 30, phase
adjusting sections configured to adjust the optical phase of guided
light independently from the phase shifting section 10 by changing
the refractive index of at least one of the optical waveguide 2B
and the optical waveguide 3B. The refractive index of the phase
adjusting sections can be changed by applying a voltage to at least
one of the third and fourth upper electrodes on the third and
fourth optical waveguide structures of the phase adjusting
sections. Because of the change in refractive index, the difference
in phase shift amount at the second end facet 1b between the
optical waveguide 2B and the optical waveguide 3B can be
compensated. Accordingly, the conditions for the operation voltage
of the phase shifting section 10 can be maintained substantially
constant.
Modification Example 5
[0109] Modification Example 5 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 9 is a plan view showing Modification Example 5 of a
Mach-Zehnder interferometer type optical modulator. FIG. 10 is a
cross-sectional view of the Mach-Zehnder interferometer type
optical modulator shown in FIG. 9 taken along line X-X.
[0110] A Mach-Zehnder interferometer type optical modulator 1F
shown in FIG. 9 differs from the Mach-Zehnder interferometer type
optical modulator 1A shown in FIG. 1 in that an end facet 1c and a
distributed Bragg reflector (DBR) portion 70 are provided instead
of the second end facet 1b and the high-reflection film HR,
respectively. The DBR portion 70 is a reflecting portion according
to this modification example and constitutes part of the optical
waveguide 2B and part of the optical waveguide 3B. As shown in FIG.
10, the DBR portion 70 includes two optical waveguide structures
(fifth and sixth optical waveguide structures) each including a
lower cladding layer 73 on the semiconductor substrate 4, a core
layer 74 on the lower cladding layer 73, and an upper cladding
layer 75 on the core layer 74. The core layer 74 has a diffraction
grating G formed therein.
[0111] The DBR portion 70 has a predetermined reflection band
determined by the period of the diffraction grating. Accordingly,
only a light having a predetermined wavelength can be reflected
from among the light that has propagated through the optical
waveguides 2B and 3B. In other words, the Mach-Zehnder
interferometer type optical modulator 1F can selectively modulate a
light with the predetermined wavelength. Thus, the noise of the
modulated light can be reduced and the S/N ratio can be improved.
The DBR portion 70 may be prepared by forming a diffraction grating
G in the core layer.
[0112] Thus, the DBR portion 70 can be formed within the
semiconductor process of manufacturing a Mach-Zehnder
interferometer type optical modulator.
Modification Example 6
[0113] Modification Example 6 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 11 is a plan view showing Modification Example 6 of a
Mach-Zehnder interferometer type optical modulator. FIG. 12 is a
cross-sectional view of the Mach-Zehnder interferometer type
optical modulator shown in FIG. 11 taken along line XII-XII.
[0114] A Mach-Zehnder interferometer type optical modulator 1G
shown, in FIGS. 11 and 12 differs from the Mach-Zehnder
interferometer type optical modulator 1A shown in FIG. 1 in that an
end facet 1c and a DBR portion 80 are provided instead of the
second end facet 1b and the high-reflection film HR. The DBR
portion 80 is a reflecting portion in this modification example.
The DBR portion 80 has a diffraction grating structure including
semiconductor portions 88 and dielectric portions D alternately and
periodically arranged in the waveguiding direction (Y direction in
the example shown in FIGS. 11 and 12). The DBR portion 80 includes
two optical waveguide structures (seventh and eight optical
waveguide structures) each including dielectric portions D and
semiconductor portions 88 including a lower cladding layer 83 on
the semiconductor substrate 4, a core layer 84 on the lower
cladding layer 83, and an upper cladding layer 85 on the core layer
84. The dielectric portions D are composed of a dielectric
material.
[0115] The DBR portion 80 of the Mach-Zehnder interferometer type
optical modulator 1G of Modification Example 6 has a predetermined
reflection band determined by the period of the diffraction grating
constituted by the semiconductor portions 88 and the dielectric
portions D. Accordingly, only a light having a predetermined
wavelength can be reflected among the light propagating through the
optical waveguides 2B and 3B. Moreover, the difference in
refractive index between the semiconductor portions and the
dielectric portions of the diffraction grating can be increased.
Thus, a high reflectivity of more than 90% can be easily
achieved.
[0116] Examples of the dielectric material constituting the
dielectric portions D include dielectric materials having a low
refractive index such as benzocyclobutene resin (BCB), silicon
oxide, silicon nitride, and polyimide resin. The dielectric
portions D need not be filled with these dielectric materials and
may be left empty. In other words, semiconductor portions 88 and
air gaps between the semiconductor portions 88 may constitute the
DBR portion 80.
Modification Example 7
[0117] Modification Example 7 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 13 is a plan view showing Modification Example 7 of a
Mach-Zehnder interferometer type optical modulator. FIG. 12 is also
a cross-sectional view of the Mach-Zehnder interferometer type
optical modulator shown in FIG. 13 taken along line XII-XII.
[0118] A Mach-Zehnder interferometer type optical modulator 1H
shown in FIG. 13 differs from the Mach-Zehnder interferometer type
optical modulator 1A shown in FIG. 1 in that an end facet 1c and a
DBR portion 90 including semiconductor portions 88 and dielectric
portions D alternately arranged are provided instead of the second
end facet 1b and the high-reflection film HR. The optical
waveguides 2B and 3B include the DBR portion 90 optically connected
to the phase shifting section 10. The DBR portion 90 is the
reflecting portion of this modification example. The structure of
the DBR portion 90 of Modification Example 7 is basically the same
as that of the DBR portion 80 of Modification example 6. However,
as shown in FIG. 13, the semiconductor portions 88 are not
individually formed in the optical waveguides 2B and 3B but are
integrated. Accordingly, the cross-sectional structure of the
Mach-Zehnder interferometer type optical modulator 1H is the same
as that shown in FIG. 12.
[0119] The height of the DBR portion in the direction perpendicular
to the semiconductor substrate 4 of the semiconductor portions (Z
direction in the drawing) is, for example, 3 to 4 .mu.m and the
width thereof in the waveguiding direction (Y direction in the
drawing) is several hundred nanometers, which is relatively small.
When the semiconductor portions 88 of the optical waveguide 2B are
separately formed from the semiconductor portions 88 of the optical
waveguide 3B as in Modification Example 6, the mechanical strength
of the DBR portion is low. In contrast, according to the structure
of Modification Example 7, the semiconductor portions of the
optical waveguide 2B are integrated with the semiconductor portions
of the optical waveguide 3B, and thus the width in the direction (X
direction in the drawing) perpendicular to the waveguiding
direction (Y direction in the drawing) is increased. Thus, the
mechanical strength of the DBR portion can be improved.
Modification Example 8
[0120] Modification Example 8 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 14 is a plan view showing Modification Example 8 of a
Mach-Zehnder interferometer type optical modulator.
[0121] A Mach-Zehnder interferometer type optical modulator 1I
shown in FIG. 14 differs from the Mach-Zehnder interferometer type
optical modulator 1A shown in FIG. 1 in that the optical waveguides
2B and 3B further include a DBR portion 91 between the phase
shifting section 10 and the second end facet 1c, the DBR portion 91
including alternately arranged semiconductor portions 88 and
dielectric portions D. The DBR portion 91 is a reflecting portion
of this modification example. The structure of the DBR portion 91
of Modification Example 8 is basically the same as that of the DBR
portion 90 of Modification example 7. However, as shown in FIG. 14,
the DBR portion 91 of Modification Example 8 has semiconductor
portions 88 that extend beyond the optical waveguides 2B and 3B in
the direction (X direction in the drawing) perpendicular to the
waveguiding direction (Y direction in the drawing). According to
the structure of Modification Example 8, the width of the
semiconductor portions in the X direction is larger than that of
Modification Example 7. Thus, the mechanical strength of the DBR
portion can be further enhanced.
Modification Example 9
[0122] Modification Example 9 of the Mach-Zehnder interferometer
type optical modulator 1A described above will now be described.
FIG. 15 is a plan view showing Modification Example 9 of a
Mach-Zehnder interferometer type optical modulator. FIG. 16 is a
cross-sectional view of the Mach-Zehnder interferometer type
optical modulator shown in FIG. 15 taken along line XVI-XVI.
[0123] A Mach-Zehnder interferometer type optical modulator 1J
shown in FIGS. 15 and 16 differs from the Mach-Zehnder
interferometer type optical modulator 1A shown in FIG. 1 in that
the optical waveguides 2B and 3B further include a DBR portion 80
and an optical detection region P between the phase shifting
section 10 and the second end facet 1c. The optical detection
region P has a function of monitoring guided light. The optical
detection region P is disposed between the DBR portion 80 and the
second end facet 1c. The DBR portion 80 of Modification Example 9
may be the DBR portion 80 of Modification Example 6. The DBR
portion 80 is the reflecting portion according to this modification
example.
[0124] As shown in FIG. 16, the optical detection region P is
joined to the DBR portion 80 by a butt joint method. A lower
cladding layer 13f, a light-absorbing layer Q, an upper cladding
layer 15f, a contact layer 16f, and an electrode 11f are stacked on
the semiconductor substrate 4 in that order in the optical
detection region P. The light-absorbing layer Q may be composed of
a material that can be used to form the first core layer 14a of the
phase shifting section 10. A material having a band gap energy that
can absorb the light emitted from the DBR portion 80 is used in the
light-absorbing layer Q.
[0125] Most of the guided light input to the Mach-Zehnder
interferometer type optical modulator 1J from the first end facet
1a is reflected by the DBR portion 80 toward the first end facet 1a
side. However, the rest of the light passes through the DBR portion
80 and enters the optical detection region P. The light that has
entered the optical detection region P is absorbed with the
light-absorbing layer Q and converted into a photocurrent. Since
the photocurrent is proportional to the power of the guided light,
the optical detection region P can be used as a power monitor for
the guided light.
[0126] In Modification Example 9, a structure in which the optical
detection region P is jointed with the DBR portion 80 by the butt
joint method is shown. Alternatively, the DBR portion may be the
same as the DBR portion 70 of Modification Example 5, the DBR
portion 90 of Modification Example 7, or the DBR portion 91 of
Modification Example 8.
[0127] Although the principle of the present invention has been
described through preferred embodiments above, persons skilled in
the art would naturally recognize that modifications and
alterations of details are possible without departing from the
principle of the present invention. Accordingly, the scope of the
protection shall be extended to all modifications and alterations
within the scope of the claims and the spirit thereof.
* * * * *