U.S. patent application number 16/708882 was filed with the patent office on 2020-07-09 for optical wavelength conversion device and method for manufacturing the same.
The applicant listed for this patent is Sumitomo Electric Industries, Ltd.. Invention is credited to Shigehiro NAGANO.
Application Number | 20200218127 16/708882 |
Document ID | / |
Family ID | 71104221 |
Filed Date | 2020-07-09 |
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United States Patent
Application |
20200218127 |
Kind Code |
A1 |
NAGANO; Shigehiro |
July 9, 2020 |
OPTICAL WAVELENGTH CONVERSION DEVICE AND METHOD FOR MANUFACTURING
THE SAME
Abstract
An object is to provide, for example, a method for manufacturing
an optical wavelength conversion device having a structure that
enables efficient formation of crystal regions on the surface of,
or inside, an amorphous material. An amorphous main body is
intermittently irradiated with a first laser beam for generating a
high-density excited electron region inside the main body and a
second laser beam for heating the high-density excited electron
region, with respective focus regions of the first and second laser
beams overlapping each other. During the intermittent irradiation
with the first and second laser beams, the relative position of the
main body and the overlapping focus region of the first and second
laser beams are varied. This enables part of the main body where
the overlapping focus region moves to serve as a heat source for
forming a crystal region.
Inventors: |
NAGANO; Shigehiro; (Osaka,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sumitomo Electric Industries, Ltd. |
Osaka |
|
JP |
|
|
Family ID: |
71104221 |
Appl. No.: |
16/708882 |
Filed: |
December 10, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/353 20130101;
G02F 2001/3548 20130101; G02F 1/365 20130101; G02F 1/3551 20130101;
G02F 1/3544 20130101 |
International
Class: |
G02F 1/355 20060101
G02F001/355; G02F 1/35 20060101 G02F001/35; G02F 1/365 20060101
G02F001/365 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 9, 2019 |
JP |
2019-002041 |
Claims
1. A method for manufacturing an optical wavelength conversion
device, comprising: a preparing step of preparing a main body made
of an amorphous material; a first irradiating step of irradiating
the main body with a first laser beam focused on a surface of or
inside the main body and exciting electrons in a focus region of
the first laser beam, the first laser beam being a femtosecond
laser beam having a wavelength outside an absorption wavelength
band of the main body; a second irradiating step of irradiating the
main body with a second laser beam focused to overlap the focus
region of the first laser beam and heating the focus region of the
first laser beam, the second laser beam being a continuous wave
laser beam or a pulsed laser beam with a pulse width of one
picosecond or more, the second laser beam being a laser beam
having, outside the focus region of the first laser beam, a
wavelength outside the absorption wavelength band of the main body;
and a scanning step of varying a relative position of the main body
and an overlapping focus region of the first and second laser beams
while the first and second irradiating steps are being
intermittently carried out in a synchronized manner.
2. The method according to claim 1, wherein the main body is made
of BaO-TiO.sub.2-GeO.sub.2-SiO.sub.2-based glass or
SrO-TiO.sub.2-SiO.sub.2-based glass.
3. The method according to claim 2, wherein the main body includes
a metal of any of the lanthanoid series, actinoid series, and group
4 to group 12 as an additive.
4. The method according to claim 1, wherein the first laser beam
includes any of a laser beam output from a titanium-sapphire laser,
a laser beam obtained by converting a wavelength of the laser beam
output from the titanium-sapphire laser, a laser beam output from a
ytterbium-doped fiber laser, and a laser beam obtained by
converting a wavelength of the laser beam output from the
ytterbium-doped fiber laser.
5. The method according to claim 1, wherein the second laser beam
includes a laser beam output from any of a carbon dioxide laser, an
ytterbium-doped fiber laser, and a semiconductor laser.
6. The method according to claim 1, further comprising a processing
step of forming a channel waveguide structure in the main body
before the first and second irradiating steps or after the scanning
step, the channel waveguide structure having an optical axis
extending along a direction in which the overlapping focus region
of the first and second laser beams moves.
7. An optical wavelength conversion device comprising: a main body
configured to allow light to propagate therein; and a plurality of
crystal regions arranged inside the main body along a propagation
direction of the light, wherein the plurality of crystal regions
each have a spontaneous polarization oriented along the propagation
direction.
8. The optical wavelength conversion device according to claim 7,
wherein adjacent ones of the plurality of crystal regions are
arranged, with portions thereof having the respective spontaneous
polarizations oriented along the propagation direction in contact
with each other.
9. The optical wavelength conversion device according to claim 7,
wherein adjacent ones of the plurality of crystal regions are
spaced apart, with an amorphous region therebetween.
10. The optical wavelength conversion device according to claim 7,
wherein the main body includes a substrate with a channel waveguide
structure having an optical axis extending along the propagation
direction.
11. The optical wavelength conversion device according to claim 7,
wherein the main body includes an optical fiber having a central
axis extending along the propagation direction; the optical fiber
includes a core containing and extending along the central axis, an
optical cladding surrounding the core and having a refractive index
lower than a refractive index of the core, and a physical cladding
surrounding the optical cladding and having a refractive index
lower than the refractive index of the core; and the plurality of
crystal regions each form at least part of an optical waveguide
region including the core and the optical cladding.
12. The optical wavelength conversion device according to claim 7,
wherein the main body is made of a material mainly composed of
BaO-TiO.sub.2-GeO.sub.2-SiO.sub.2-based glass or
SrO-TiO.sub.2-SiO.sub.2-based glass, and the plurality of crystal
regions are fresnoite crystals.
13. The optical wavelength conversion device according to claim 12,
wherein the main body includes a metal of any of the lanthanoid
series, actinoid series, and group 4 to group 12 as an
additive.
14. The optical wavelength conversion device according to claim 7,
wherein the plurality of crystal regions each include a first
crystal sub-region having a spontaneous polarization radially
oriented in a direction perpendicular to the propagation direction,
and second crystal sub-regions located at both ends of the first
crystal sub-region in the propagation direction, the second crystal
sub-regions having respective spontaneous polarizations oriented
along the propagation direction, the spontaneous polarizations each
oriented in a direction different from the spontaneous polarization
of the first crystal sub-region; when the plurality of crystal
regions are arranged in such a manner that the light propagates
from one of the second crystal sub-regions to the other of the
second crystal sub-regions, a first interface between the first
crystal sub-region and the one of the second crystal sub-regions
and a second interface between the first crystal sub-region and the
other of the second crystal sub-regions are alternately arranged
along the propagation direction; and a repetitive structure is a
structure where an interval between first interfaces in two
adjacent ones of the crystal regions along the propagation
direction or an interval between second interfaces in the two
adjacent ones of the crystal regions is defined as one period, and
the repetitive structure has a constant period, a chirp period, a
period formed by combining a plurality of different constant
periods, or a period based on a Fibonacci sequence or Barker
sequence.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to an optical wavelength
conversion device and a method for manufacturing the same.
2. Description of the Related Art
[0002] Ferroelectric optical crystals, such as a LiNbO.sub.3 (LN)
crystal, a KTiOPO.sub.4 (KTP) crystal, a LiB.sub.3O.sub.5 (LBO)
crystal, and a .beta.-BaB.sub.2O.sub.4 (BBO) crystal, are materials
typically used in second-order nonlinear devices that have been
developed in a wide range of application fields based on wavelength
conversion. In the field of laser processing, these crystals are
used to shorten the wavelengths of optical fiber lasers using
second harmonic generation (SHG). Since beam spot diameters can be
reduced, the crystals described above are used in fine laser
processing. In the field of optical communications, the crystals
described above are used in optical devices that perform
simultaneous multiple wavelength conversion of C-band wavelength
division multiplexing (WDM) signals to L-band WDM signals for
effective use of wavelength resources of the WDM signals. In the
field of measurement, where terahertz spectroscopy that enables
observations of intermolecular vibrations caused by hydrogen
bonding has attracted attention, the crystals described above are
used in light sources that generate terahertz light. Recently,
compound semiconductor crystals, such as GaAs, GaP, GaN, CdTe,
ZnSe, and ZnO, have attracted attention as materials for
second-order nonlinear devices, because they have large
second-order nonlinear optical constants and there have been
significant advances in the technology of making periodically-poled
structures that are essential for second-order nonlinear
devices.
[0003] Wavelength conversion techniques can be divided into two
types: angle phase matching, and quasi-phase matching (QPM) based
on periodic poling. In particular, the quasi-phase matching
enables, by adjusting the periodic poling pitch, generation of a
plurality of phase-matched wavelengths and wavelength conversion
over the entire transparent region of the material. Additionally,
with the quasi-phase matching, which is free from walk-off angles
that are inevitable when using angle phase matching, it is possible
to achieve high beam quality and increase the interaction length.
The quasi-phase matching thus enables efficient use of wavelength
resources and reduction of coupling loss in optical communication,
and thus is an effective technique suitable for use in the fields
of laser processing and measurement due to the high beam
quality.
[0004] If the material used in the second-order nonlinear device is
a single-crystal material, however, the wavelength conversion using
the quasi-phase matching still has constraints in the forming
process and requires a complex optical system. International
Publication No. 2017/110792 proposes a technique that combines a
flexible glass forming process with wavelength conversion. The
advantage of this technique is that the substrate, which is made of
glass, can be processed into various forms, such as fibers or thin
films. That is, since wavelength conversion capabilities can be
added to various forms of substrates, user-friendly wavelength
conversion can be achieved. International Publication No.
2017/110792 described above also discloses an orientation control
technique which involves aligning crystals in a region irradiated
with a laser beam under application of an electric field.
[0005] As a simple and selective crystallization technique,
International Publication No. 2018/123110 proposes a selective
crystallization technique using laser annealing. This technique
involves irradiating precursor glass with a laser beam that has a
wavelength in the absorption wavelength band of the precursor
glass. This laser irradiation causes local heat application
resulting from absorption of light in the laser-irradiated regions,
or causes local heat application to a material surface through a
film coated with an absorbing material, and thus enables formation
of crystal regions having local spontaneous polarizations.
SUMMARY OF THE INVENTION
[0006] The present invention provides a manufacturing method that
prevents an increase in the number of manufacturing steps, involves
no complex operations for optimizing manufacturing conditions and
facilities, and yet offers greater flexibility in forming crystal
regions on the surface of, or inside, an amorphous material to form
an optical wavelength conversion device. The present invention also
provides an optical wavelength conversion device obtained by the
manufacturing method.
[0007] A method for manufacturing an optical wavelength conversion
device according to the present disclosure includes a preparing
step, a first irradiating step, a second irradiating step, and a
scanning step. The preparing step prepares a main body made of an
amorphous material to form an optical wavelength conversion device.
The first irradiating step irradiates the main body with a first
laser beam focused on the surface of, or inside, the main body and
excites electrons in the focus region of the first laser beam. A
femtosecond (fs) laser beam is used as the first laser beam. The fs
laser beam has a wavelength outside the absorption wavelength band
of the main body, or a wavelength at which the absorption of light
into the main body can be kept at a low level. The second
irradiating step irradiates the main body with a second laser beam
focused to overlap the focus region of the first laser beam, and
heats the focus region of the first laser beam. A pulsed laser beam
with a pulse width of 1 picosecond (ps) or more, or a continuous
wave (CW) laser beam is used as the second laser beam. Outside the
focus region of the first laser beam, either the pulsed laser beam
or the CW laser beam has a wavelength outside the absorption
wavelength band of the main body, or has a wavelength at which the
absorption of light into the main body can be kept at a low level.
The scanning step varies the relative position of the main body and
the overlapping focus region of the first and second laser beams
while the first and second irradiating steps are being
intermittently carried out in a synchronized manner.
[0008] In the present disclosure, "wavelength outside the
absorption wavelength band" and "wavelength at which the absorption
of light can be kept at a low level" refer to a wavelength at which
the absorption coefficient is 0.01/cm or less. The focus region of
the first laser beam refers to a region (high-density excited
electron region) where excited electrons are present at high
densities, with the focus point of the first laser beam at the
center, and is defined as a region where the density of excited
electrons is 10.sup.19/cm.sup.3 or more. The state where the focus
region of the first laser beam and the focus region of the second
laser beam overlap not only refers to the state where the focus
point of the first laser beam coincides with the focus point of the
second laser beam, but also refers to the state where these focus
points do not coincide. Specifically, for example, even when the
focus point of the second laser beam is located outside the
high-density excited electron region (i.e., outside the focus
region of the first laser beam), the entire or at least part of the
high-density excited electron region may be located within the
region irradiated with the second laser beam.
[0009] An optical wavelength conversion device according to the
present disclosure includes a main body configured to allow light
to propagate therein, and a plurality of crystal regions arranged
inside the main body along a propagation direction of the light.
The plurality of crystal regions each have a spontaneous
polarization oriented along the propagation direction (i.e.,
spontaneous polarization having a polarization orientation
coinciding with the propagation direction).
[0010] The present disclosure enables efficient formation of
crystal regions, and provides an optical wavelength conversion
device capable of highly efficient wavelength conversion on the
surface of, or inside, the main body of any of various shapes, such
as a bulky shape and a fiber shape.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a conceptual diagram for explaining a main
structure of an optical wavelength conversion device and a
principle of how a crystal region is formed, according to an
embodiment of the present disclosure.
[0012] FIG. 2 is a conceptual diagram for explaining a relation
between crystallization and temperature during direct laser
irradiation of an amorphous material.
[0013] FIG. 3 is a conceptual diagram for explaining a method for
manufacturing an optical wavelength conversion device according to
the present disclosure.
[0014] FIG. 4 is a conceptual diagram for explaining another method
for manufacturing an optical wavelength conversion device according
to the present disclosure.
[0015] FIG. 5 is a conceptual diagram illustrating, as an example
of the optical wavelength conversion device according to the
present disclosure, a repetitive structure where a plurality of
crystal regions are arranged alternately with amorphous regions in
a main body.
[0016] FIG. 6 is a conceptual diagram illustrating, as another
example of the optical wavelength conversion device according to
the present disclosure, a repetitive structure where a plurality of
crystal regions are continuously arranged in the main body.
[0017] FIG. 7 is a conceptual diagram for explaining the order of
polarizations in the structure illustrated in FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] An optical wavelength conversion device and a method for
manufacturing the optical wavelength conversion device according to
embodiments of the present invention will now be described in
detail with reference to the attached drawings. Note that the
present invention is not limited to the embodiments described
herein. The present invention is defined by the appended claims,
and all changes made within the appended claims and meanings and
scopes equivalent thereto are intended to be embraced by the
present invention. The same elements are denoted by the same
reference numerals throughout the drawings, and redundant
description will be omitted.
[0019] The technique disclosed in International Publication No.
2017/110792 involves the step of applying an electric field, and
this requires preparation of electrodes. Since a voltage is applied
at short intervals, a special attention needs to be paid during
application of a high voltage to avoid dielectric breakdown. The
technique disclosed in International Publication No. 2017/110792
thus requires many manufacturing steps and increases the difficulty
of manufacture. In the technique disclosed in International
Publication No. 2018/123110, the crystallization inside the
material is highly dependent on the amount of light absorption.
Even when the wavelength of a laser beam with which to irradiate
the material is set to be short, if the material does not absorb a
sufficient amount of light, it is difficult to achieve flexible
formation of crystal regions inside the material only by optimizing
the laser irradiation conditions and the light-condensing optical
system.
[0020] FIG. 1 is a conceptual diagram for explaining a main
structure of an optical wavelength conversion device and a
principle of how a crystal region is formed, according to an
embodiment of the present disclosure. FIG. 2 is a conceptual
diagram for explaining a relation between crystallization and
temperature during direct laser irradiation of an amorphous
material.
[0021] A method for manufacturing an optical wavelength conversion
device according to the present disclosure may use a glass
containing SiO.sub.2 as an amorphous material for a main body to
form the optical wavelength conversion device. The amorphous main
body is irradiated with an fs laser beam (first laser beam) having
a wavelength outside the absorption wavelength band of the main
body, or a wavelength at which the level of absorption of light
into the main body is low, and also with a laser beam (second laser
beam) having a wavelength outside the absorption wavelength band of
the main body or a wavelength at which the level of absorption of
light into the main body is low. The second laser beam is either a
pulsed laser beam with a pulse width of 1 ps or more, or a CW laser
beam. The first laser beam and the second laser beam are applied to
the main body in such a manner as to overlap in the same focus
region. At this point, when the pulsed laser beam with a pulse
width of 1 ps or more, or the CW laser beam, is preferentially
absorbed in a high-density excited electron region temporarily
generated in the focus region of the fs laser beam, heat is
generated in the high-density excited electron region. The
manufacturing method of the present disclosure crystallizes the
neighboring region of this heated high-density excited electron
region (heat-generating region), and enables flexible formation of
one or more crystal regions on the surface of, or inside, the main
body.
[0022] Referring to FIG. 1, a main body 10 is made of an amorphous
material, and light propagates in the main body 10 along an optical
axis AX. For example, if the main body 10 is a substrate having a
channel waveguide structure, the optical axis of the channel
waveguide coincides with the optical axis AX. Similarly, if the
main body 10 is an optical fiber, the central axis of the optical
fiber coincides with the optical axis AX. Materials that can be
used to form the main body 10 include a
BaO-TiO.sub.2-GeO.sub.2-SiO.sub.2-based glass or a
SrO-TiO.sub.2-SiO.sub.2-based glass. In the main body 10 made of
these materials, a radially ordered polarization structure composed
of fresnoite crystals (Sr.sub.2TiSi.sub.2O.sub.8,
Ba.sub.2TiGe.sub.2O.sub.8) can be easily obtained by laser
irradiation. A metal of any of the lanthanoid series, actinoid
series, and group 4 to group 12 may be added to the main body 10
made of one of the materials described above. This enhances
absorption of the laser beam into the main body 10, and enables
more efficient formation of the ordered polarization structure.
[0023] In the present disclosure, the main body 10 is irradiated
with two different types of laser beams L1 and L2 acting
differently on the main body 10. The laser beam L1 (first laser
beam) is applied to the main body 10 in such a manner that the
focus region of the laser beam L1 is located on the surface of, or
inside, the main body 10. The laser beam L1 is a laser beam for
generating a high-density excited electron region 110 in the main
body 10 (see, e.g., Nature Photonics 2, 219-225 (2008) by Rafael R.
Gattass & Eric Mazur) and includes an fs laser beam having a
wavelength outside the absorption wavelength band of the main body
10 or a wavelength at which the absorption of light into the main
body 10 can be kept at a low level. Examples of the laser beam L1
include a laser beam output from a titanium-sapphire (Ti:S) laser,
a laser beam output from a fiber laser (e.g., ytterbium-doped
(Yb-doped) fiber laser), and a laser beam output from a wavelength
conversion laser (with a wavelength of 400 nm to 550 nm) using
these laser sources. All the laser beams described here are pulsed
laser beams with a pulse width of 900 fs or less.
[0024] On the other hand, the laser beam L2 (second laser beam) is
applied to the main body 10 in such a manner that the focus region
of the laser beam L2 overlaps the focus region of the laser beam
L1. The laser beam L2 is a laser beam having the function of
heating the high-density excited electron region 110 of the main
body 10. Outside the focus region of the laser beam L1, the laser
beam L2 has a wavelength outside the absorption wavelength band of
the main body 10, or has a wavelength at which the absorption of
light into the main body 10 can be kept at a low level. The laser
beam L2 includes a pulsed laser beam with a pulse width of 1 ps or
more, or a CW laser beam. Examples of the light source for
outputting the laser beam L2 include a gas laser (e.g., carbon
dioxide (CO.sub.2) laser), a fiber laser (e.g., Yb-doped fiber
laser), and a semiconductor laser. The laser beam L2 output from
any of these light sources includes a pulsed laser beam with a
pulse width of 1 ps or more (preferably with a pulse width of 1
nanosecond (ns) or more), or a CW laser beam.
[0025] The high-density excited electron region 110 illustrated in
FIG. 1 is a region where electrons temporarily excited by
irradiation with the laser beam L1 are present at high densities.
When the laser beam L2 is focused toward the high-density excited
electron region 110, the optical energy of the laser beam L2 is
preferentially and selectively absorbed in the high-density excited
electron region 110. This optical-energy absorbing region generates
heat, and functions as a heat-generating region (heat source) for
forming a crystal region 100 (see, e.g., Applied Physics Letters,
Vol. 113, 061101/1-4 (2018) by Yusuke Ito, et al.).
[0026] In the present disclosure, the high-density excited electron
region 110 in the main body 10 is shifted along the optical axis AX
by moving at least the laser beams L1 and L2 or the main body 10 in
the direction indicated by arrow S1 in FIG. 1. This means that the
region 110 indicated by diagonal lines in FIG. 1 is where a
high-density excited electron region has been previously generated
by irradiation with the laser beam L1 and has functioned as a
heat-generating region in the process of formation of the crystal
region 100 by absorbing the optical energy of the laser beam L2.
The crystal region 100 formed in the main body 10 is a neighboring
region of the high-density excited electron region 110.
Specifically, the crystal region 100 is a region crystallized by
heat generated in parts of the high-density excited electron region
110 during irradiation with the laser beams L1 and L2 that are
moved relative to each other along the direction indicated by arrow
S1.
[0027] The crystal region 100 formed as described above is composed
of a first crystal sub-region 100A having a spontaneous
polarization A radially orientated in the direction perpendicular
to the optical axis AX, and second crystal sub-regions 100B1 and
100B2 located at both ends of the first crystal sub-region 100A
along the optical axis AX. The second crystal sub-regions 100B1 and
100B2 have spontaneous polarizations B1 and B2, respectively.
Unlike the spontaneous polarization A in the first crystal
sub-region 100A, the spontaneous polarizations B1 and B2 are
orientated along the optical axis AX (scanning direction of the
laser beam L1).
[0028] The position of an interface 120A between the first crystal
sub-region 100A and the second crystal sub-region 100B1 can be
identified as one end of the first crystal sub-region 100A, that
is, as the irradiation start position of the laser beam L1.
Similarly, the position of an interface 120B between the first
crystal sub-region 100A and the second crystal sub-region 100B2 can
be identified as the other end of the first crystal sub-region
100A, that is, as the irradiation end position of the laser beam
L1.
[0029] Particularly in the optical wavelength conversion device of
the present disclosure, a plurality of crystal regions 100, each
having the structure illustrated in FIG. 1, are arranged along the
optical axis AX in the main body 10. In the plurality of crystal
regions 100 arranged along the optical axis AX in the main body 10,
the interfaces 120A and 120B, which define the positions of both
ends of each first crystal sub-region 100A, are alternately
arranged along the optical axis AX. When the plurality of crystal
regions 100 are arranged in the main body 10, an interval between
interfaces 120A in two adjacent ones of the crystal regions 100
along the optical axis AX, or an interval between interfaces 120B
in two adjacent ones of the crystal regions 100 along the optical
axis AX, is defined as one period of the repetitive structure. This
repetitive structure preferably has a constant period, a chirp
period, a period including a plurality of different constant
periods, or a period based on a Fibonacci sequence or Barker
sequence.
[0030] The amount of heat generation in the region where the
optical energy of the laser beam L2 is absorbed (absorption region)
is dependent on the duration of irradiation with the laser beam L2.
As the amount of heat generation increases, the temperature in the
neighboring region around the absorption region also increases
(from a crystal nucleation threshold T1 to a crystal growth
threshold T2 as shown in FIG. 2). The neighboring region can be
crystallized by controlling the amount of heat generation in the
absorption region such that the temperature in the neighboring
region is lower than or equal to a damage (or melting) threshold
T3.
[0031] FIG. 2 is a diagram for explaining a relation between
crystallization and temperature during direct laser irradiation of
an amorphous material, which is a typical example of laser
irradiation. The crystal region 100 illustrated in FIG. 2 coincides
with a cross-section of the main body 10 orthogonal to the optical
axis AX illustrated in FIG. 1. The crystal region 100 has the
spontaneous polarization A radially oriented as illustrated in FIG.
2. Curves G1 to G3 in FIG. 2 indicate that when an amorphous
material (target) is irradiated with a laser beam, the temperature
in the irradiated region is highest on the optical axis of the
laser beam, and decreases with increasing distance from the optical
axis of the laser beam in the radial direction.
[0032] At the stage of curve G1, only the center temperature in the
irradiated region has reached the crystal nucleation threshold T1
and the temperature in the other region has not yet reached the
crystal nucleation threshold T1. Crystal nuclei are formed only in
the center of the irradiated region, and the spontaneous
polarization is randomly oriented at this point.
[0033] As continuous or intermittent laser irradiation continues,
the overall temperature distribution rises and the center
temperature in the irradiated region reaches the crystal growth
threshold T2 as indicated by curve G2. This allows the crystals to
start growing at the crystal nuclei. The crystals grow in
accordance with the random orientation of the spontaneous
polarization. The crystal nuclei growing toward the center of the
irradiated region collide with each other and stop growing. This
makes the orientation toward the outer region where the crystals
can grow dominant. Therefore, the final orientation of the
spontaneous polarization A is mainly away from the center of the
irradiated region (i.e., from the optical axis of the laser beam)
along the radial direction.
[0034] The continuous or intermittent laser irradiation continues,
and when the temperature in and around the center of the irradiated
region exceeds the damage threshold T3 as indicated by curve G3,
the target melts in and around the center. This means that a
perforation (processing mark) 101 is formed in the center of the
crystal region. The crystal region 100 having an annular shape is
thus formed, which has the spontaneous polarization A oriented
radially.
[0035] FIG. 3 is a conceptual diagram for explaining a method for
manufacturing an optical wavelength conversion device according to
the present disclosure. As the main body 10, a waveguide substrate
10A having a channel waveguide 11 extending along the optical axis
AX is prepared (preparing step). A first light source 20A outputs
the laser beam L1 (fs laser beam) for generating a high-density
excited electron region on the surface of, or inside, the waveguide
substrate 10A, and the waveguide substrate 10A is irradiated with
the laser beam L1 (first irradiating step). A second light source
20B outputs the laser beam L2 (which is a pulsed laser beam with a
pulse width of 1 ps or more, or a CW laser beam) for heating part
of the waveguide substrate 10A, and the waveguide substrate 10A is
irradiated with the laser beam L2 (second irradiating step). The
laser beam L1 and the laser beam L2 are coaxially applied to the
waveguide substrate 10A. That is, the optical path of the laser
beam L1, extending from the first light source 20A to the waveguide
substrate 10A, and the optical path of the laser beam L2, extending
from the second light source 20B to the waveguide substrate 10A,
are provided with a light-condensing optical system 30 and a half
mirror 40 that are shared by the laser beams L1 and L2. This
coaxial irradiation system is advantageous in that it can be
configured easily.
[0036] The first irradiating step and the second irradiating step
are carried out in a synchronized manner to enable intermittent
irradiation with the laser beam L1 and the laser beam L2. During
the laser irradiation, the laser beam L1 output from the first
light source 20A is reflected by the half mirror 40 toward the
light-condensing optical system 30. After passing through the
light-condensing optical system 30, the laser beam L1 is focused
near the surface of the waveguide substrate 10A. The high-density
excited electron region 110 is generated in the focus region of the
laser beam L1. At the same time, the laser beam L2 output from the
second light source 20B travels through the half mirror 40 toward
the light-condensing optical system 30. After passing through the
light-condensing optical system 30, the laser beam L2 is focused to
overlap the high-density excited electron region 110. The optical
energy of the laser beam L2 is efficiently absorbed in the
high-density excited electron region 110, which functions as a
heat-generating region to form the crystal region 100 in the
channel waveguide 11.
[0037] While the first and second irradiating steps are being
intermittently carried out in a synchronized manner, at least the
waveguide substrate 10A or the coaxial irradiation system for the
laser beams L1 and L2 moves along the direction indicated by arrow
S2. This enables a plurality of crystal regions 100 to be formed
along the optical axis AX of the channel waveguide 11 in the
waveguide substrate 10A (scanning step).
[0038] The crystal regions 100 are formed by one scan in this
example, but may be formed by multiple scans. In the latter case,
the initial scan involves using the laser beam L2 with lower power
to form crystal nuclei at the stage of curve G1 in FIG. 2, and the
subsequent scans involve using the laser beam L2 with higher power
to enable the crystal nuclei to grow at the stage of curve G2 or G3
in FIG. 2. The channel waveguide 11 may be formed in the waveguide
substrate 10A before the first and second irradiating steps, or may
be formed in the waveguide substrate 10A after the scanning step
(processing step). The channel waveguide 11 having a ridge
structure may be formed by dry etching or may be cut out by a
dicing saw.
[0039] FIG. 4 is a conceptual diagram for explaining another method
for manufacturing an optical wavelength conversion device according
to the present disclosure. In this example, an optical fiber 10B
having a central axis extending along the optical axis AX is
prepared as the main body 10 (preparing step). The optical fiber
10B includes a core 12 containing and extending along the central
axis (which coincides with the optical axis AX), an optical
cladding 13A surrounding the core 12, and a physical cladding
(jacket) 13B surrounding the optical cladding 13A. In the optical
fiber 10B structured as described above, the crystal region 100 is
formed in at least part of an optical waveguide region 130
including the core 12 and the optical cladding 13A. Specifically, a
plurality of crystal regions 100 are formed in the core 12 (i.e.,
in the entire or part of the core 12), in the optical cladding 13A
(i.e., in the entire or part of the optical cladding 13A), in a
region including part of the core 12 and part of the optical
cladding 13A, or in a region including the entire core 12 and the
entire optical cladding 13A.
[0040] The first light source 20A outputs the laser beam L1 (fs
laser beam) for generating the high-density excited electron region
110 inside the optical fiber 10B, and the optical fiber 10B is
irradiated with the laser beam L1 (first irradiating step). The
second light source 20B outputs the laser beam L2 (which is a
pulsed laser beam with a pulse width of 1 ps or more, or a CW laser
beam) for heating the high-density excited electron region 110 in
the optical fiber 10B, and the optical fiber 10B is irradiated with
the laser beam L2 (second irradiating step). In the example
illustrated in FIG. 4, the laser beam L1 and the laser beam L2
propagate along different optical paths to reach the interior of
the optical fiber 10B. That is, a light-condensing optical system
30A is disposed in the optical path of the laser beam L1 extending
from the first light source 20A to the optical fiber 10B, and a
light-condensing optical system 30B is disposed in the optical path
of the laser beam L2 extending from the second light source 20B to
the optical fiber 10B.
[0041] As in the example illustrated in FIG. 3, the first
irradiating step and the second irradiating step are carried out in
a synchronized manner to enable intermittent irradiation with the
laser beam L1 and the laser beam L2. During the laser irradiation,
the laser beam L1 output from the first light source 20A passes
through the light-condensing optical system 30A and is focused
inside the optical fiber 10B. The high-density excited electron
region 110 is generated in the focus region of the laser beam L1.
At the same time, the laser beam L2 output from the second light
source 20B passes through the light-condensing optical system 30B
and is focused to overlap the high-density excited electron region
110. The optical energy of the laser beam L2 is efficiently
absorbed in the high-density excited electron region 110, which
functions as a heat-generating region to form the crystal region
100 in the optical fiber 10B. The crystal region 100 is controlled
in the same manner as in the example illustrated in FIG. 3.
[0042] By intermittently carrying out the first and second
irradiating steps in a synchronized manner along the direction
indicated by arrow S3, a plurality of crystal regions 100 are
formed along the central axis (optical axis AX) of the optical
fiber 10B (scanning step). By focusing the laser beam L1 at a
position off the central axis and rotating the optical fiber 10B in
the direction indicated by arrow S4 in FIG. 4, the crystal regions
100 annular in cross-section can be obtained in the optical
waveguide region 130.
[0043] When one irradiation system composed of the first light
source 20A and the light-condensing optical system 30A and the
other irradiation system composed of the second light source 20B
and the light-condensing optical system 30B are moved with respect
to the optical fiber 10B, XYZ-axis stages that hold the respective
irradiation systems are moved in a synchronized manner. The two
irradiation systems in the example illustrated in FIG. 4 offer
greater flexibility in laser irradiation, because they allow
changes in the focusing conditions of the first and second light
sources 20A and 20B. That is, the focusing conditions of the laser
beam L2 can be changed in accordance with the depth of the focus
point of the laser beam L1. A mechanism may be added to any of the
examples to synchronize the pulse irradiation of the laser beam La
and the laser beam L2 at the focus point (though this mechanism is
unnecessary if the laser beam L2 is a CW laser beam). Also, in any
of the examples, the intensities of the laser beams L1 and L2 can
be adjusted in accordance with the irradiated region.
[0044] FIG. 5 is a conceptual diagram illustrating, as an example
of the optical wavelength conversion device according to the
present disclosure, a repetitive structure where a plurality of
crystal regions are arranged alternately with amorphous regions
therebetween in the main body 10. In the example illustrated in
FIG. 5, adjacent ones of the crystal regions 100 arranged along the
optical axis AX are spaced apart, with an amorphous region
therebetween. Note that FIG. 5 conceptually illustrates the crystal
regions 100 obtained after a substrate is irradiated, in an
overlapping manner, by an fs laser and a heat-generating laser with
appropriate pulse widths, optical intensities, repetition
frequencies, focusing conditions, and wavelengths.
[0045] A cylindrical portion in the center of each crystal region
100 is the high-density excited electron region 110 generated by
irradiation with the laser beam L1. By irradiating the high-density
excited electron region 110 with the laser beam L2 in an
overlapping manner, the temperature in the neighboring region
increases from T1 to T2 as in FIG. 2. This enables the crystal
region 100 reflecting the shape of the high-density excited
electron region 110 to be formed around the high-density excited
electron region 110. The crystal region 100 is cylindrical in the
example illustrated in FIG. 5. More precisely, however, the crystal
region 100 is crystallized in a long egg shape, as its shape is
dependent on the shape of the high-density excited electron region
110 that reflects the focusing conditions.
[0046] In the center portion of the cylinder representing the
crystal region 100 (corresponding to the first crystal sub-region
100A in FIG. 1), the spontaneous polarization A radially oriented
about the optical axis AX is generated. The end portions of the
crystal region 100 (corresponding to the second crystal sub-regions
100B1 and 100B2 in FIG. 1) have the respective spontaneous
polarizations B1 and B2 oriented in opposite directions along the
optical axis AX. More precisely, each spontaneous polarization is
oriented in a direction reflecting the shape of the high-density
excited electron region 110, that is, in a direction perpendicular
to the tangent to the boundary between the high-density excited
electron region 110 and its neighboring region.
[0047] In the example of FIG. 5, the plurality of crystal regions
100 are arranged with a period which is the sum of a length L of
the high-density excited electron region 110 and a distance L'
between adjacent ones of the high-density excited electron regions
110. Specifically, L is determined by the distance between the
interface 120A and the interface 120B within one crystal region
100, and L' is determined by the distance between the interface
120B within one of two adjacent crystal regions 100 and the
interface 120A within the other of the two adjacent crystal regions
100. That is, the plurality of crystal regions 100 along the
optical axis AX are arranged with a period of L+L', and this
enables high-efficiency wavelength conversion using quasi-phase
matching.
[0048] To extend the phase matching band, for example, any of the
following structures may be employed as the aforementioned
repetitive structure: an aperiodic periodically-poled structure (or
chirp period described in IEEE J. Quantum Electron., Vol. 28,
2631-2654 (1992) by Martin M. Fejer, et al.), a structure where
multiple types of periodic regions (e.g., period A1 region, period
A2 region, and period A3 region) are treated as one segment and a
plurality of such segments are arranged at given intervals (see,
IEEE J. Quantum Electron., Vol. 30, 1596-1604 (1994) by Kiminori
Mizuuchi, et al.), a periodic structure based on a Fibonacci
sequence (see, Science, Vol. 278, 843-846 (1997) by Shi-ning Zhu,
et al.), and a periodic structure based on a Barker sequence (see,
Electronics and Communications in Japan, Part 2, Vol. 78, 20-27
(1995) by Masatoshi Fujimura, et al.).
[0049] An optical device including the main body 10 illustrated in
the example of FIG. 5 receives light that is incident along the
optical axis AX. The incident light is preferably a radially
polarized vector beam. The spontaneous polarization B1 and the
spontaneous polarization B2 coincide with the propagation direction
of the light along the optical axis AX. The nonlinear optical
constant (d) is, for example, d16 or d22. However, since the
nonlinear optical constant of the main body 10, which is a
tetragonal system, is zero, unnecessary wavelength conversion does
not take place. High-efficiency wavelength conversion is thus
achievable.
[0050] FIG. 6 is a conceptual diagram illustrating, as another
example of the optical wavelength conversion device according to
the present disclosure, a repetitive structure where a plurality of
crystal regions are continuously arranged in the main body 10.
Specifically, FIG. 6 illustrates a lateral structure and a front
structure of the main body 10. FIG. 7 is a perspective view
illustrating the orientations of spontaneous polarizations formed
inside the main body 10 in the structure illustrated in FIG. 6. In
the example illustrated in FIG. 6 and FIG. 7, adjacent ones of the
plurality of crystal regions 100 along the optical axis AX are
arranged, with portions thereof having the spontaneous
polarizations B1 and B2 in contact with each other.
[0051] In the example of FIG. 6 and FIG. 7, the overall structure
of the crystal regions 100 is the same as that in the example of
FIG. 5, and the repetitive structure has a constant period of 2L,
where L is the distance between the interfaces 120A and 120B. The
distance L is the coherence length of quasi-phase matching.
However, regardless of whether the crystal regions 100 are in
contact, or are spaced apart with an amorphous region left between
adjacent ones of the crystal regions 100 as in the example of FIG.
5, the refractive indices of the crystal regions 100 and the
amorphous regions are unchanged. That is, since such arrangement
has no impact on the wavelength conversion, adjacent ones of the
crystal regions 100 may be either in contact or spaced apart.
[0052] Although the crystal regions 100 are formed inside the main
body 10 in the example of FIG. 6 and FIG. 7, the high-density
excited electron regions 110 may be exposed at the surface of the
main body 10 as in the example of FIG. 3. By focusing the laser
beams L1 and L2 on this surface, a crystal region semicircular in
cross-section is formed. In this case, the spontaneous polarization
is radially orientated in the cross-section. High-efficiency
wavelength conversion is still achievable here, when a plurality of
crystal regions 100 are linearly arranged such that adjacent
interfaces are spaced L apart and, at the same time, the main body
10 is processed into a ridge shape in such a manner as to be
parallel to the direction in which quasi-phase matching is
established (i.e., direction along the optical axis AX).
[0053] The crystal regions 100 may be formed into any shape,
regardless of whether the main body 10 is bulky, plate-shaped, or
fiber-shaped. Also, in the optical fiber 10B illustrated in the
example of FIG. 4, any of the core 12, the optical cladding 13A,
and the physical cladding 13B may be made of the material (at least
one of BaO-TiO.sub.2-GeO.sub.2-SiO.sub.2-based glass or
SrO-TiO.sub.2-SiO.sub.2-based glass) used to form the main body 10
illustrated in FIG. 1. A metal of any of the lanthanoid series,
actinoid series, and group 4 to group 12 may be added to the entire
or part of the optical waveguide region 130 including the core 12
and the optical cladding 13A. In any of the cases described above,
by irradiating with the laser beam L1 (fs laser beam) and the laser
beam L2 (pulsed laser beam or CW laser beam) in such a manner that
their focus regions overlap, a heat-generating region can be
preferentially formed and a crystal region is formed using the heat
from the heat-generating region. Also, intermittent laser
irradiation of the main body 10 enables intermittent formation of
crystal regions (i.e., formation of a plurality of crystal regions
arranged along the optical axis).
* * * * *