U.S. patent application number 17/280920 was filed with the patent office on 2021-10-07 for terahertz waveguide.
This patent application is currently assigned to JIANGSU UNIVERSITY. The applicant listed for this patent is JIANGSU UNIVERSITY. Invention is credited to Tongtong BAI, Mingyang CHEN, Hang XU, Jianquan YAO, Yuan ZHANG.
Application Number | 20210311249 17/280920 |
Document ID | / |
Family ID | 1000005851535 |
Filed Date | 2021-10-07 |
United States Patent
Application |
20210311249 |
Kind Code |
A1 |
CHEN; Mingyang ; et
al. |
October 7, 2021 |
TERAHERTZ WAVEGUIDE
Abstract
A terahertz waveguide includes an input segment, a transmission
segment and an output segment. The input segment includes an input
waveguide and an input microstructured waveguide. One end of the
input waveguide is connected with one end of the core of the input
microstructured waveguide. The transmission segment includes at
least a sub-wavelength waveguide, an air cladding surrounding the
sub-wavelength waveguide and a solid outer cladding surrounding the
air cladding. The other end of the core of the input
microstructured waveguide is connected with one end of the
sub-wavelength waveguide. The other end of the sub-wavelength
waveguide is connected with the core of the output microstructured
waveguide. One end of the solid outer cladding is connected with
the cladding of the input microstructured waveguide, and an output
segment. The output segment includes an output microstructured
waveguide and an output waveguide.
Inventors: |
CHEN; Mingyang; (Zhenjiang,
CN) ; XU; Hang; (Zhenjiang, CN) ; ZHANG;
Yuan; (Zhenjiang, CN) ; BAI; Tongtong;
(Zhenjiang, CN) ; YAO; Jianquan; (Zhenjiang,
CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JIANGSU UNIVERSITY |
Zhenjiang |
|
CN |
|
|
Assignee: |
JIANGSU UNIVERSITY
Zhenjiang
CN
|
Family ID: |
1000005851535 |
Appl. No.: |
17/280920 |
Filed: |
June 18, 2019 |
PCT Filed: |
June 18, 2019 |
PCT NO: |
PCT/CN2019/091643 |
371 Date: |
March 29, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/262 20130101;
G02B 6/02366 20130101 |
International
Class: |
G02B 6/02 20060101
G02B006/02; G02B 6/26 20060101 G02B006/26 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2019 |
CN |
201910466568.5 |
Claims
1. A terahertz waveguide, comprising: an input segment, wherein the
input segment comprises an input waveguide; and an input
microstructured waveguide, and one end of the input waveguide is
connected with a first end of a core of the input microstructured
waveguide; a transmission segment, wherein the transmission segment
comprises a sub-wavelength waveguide, an air cladding surrounding
the sub-wavelength waveguide and a solid outer cladding surrounding
the air cladding, a second end of the core of the input
microstructured waveguide is connected with a first end of the
sub-wavelength waveguide, and a first end of the solid outer
cladding is connected with a cladding of the input microstructured
waveguide; and an output segment, wherein the output segment
comprises an output microstructured waveguide and an output
waveguide, a second end of the sub-wavelength waveguide is
connected with a first end of a core of the output microstructured
waveguide, a second end of the solid outer cladding is connected
with a cladding of the output microstructured waveguide, a second
end of the core of the output microstructured waveguide is
connected with one end of the output waveguide; wherein a diameter
de of the sub-wavelength waveguide satisfies
d.sub.c<.lamda..sub.0, wherein .lamda..sub.0 is an operating
wavelength, and the claddings of the input microstructured
waveguide and the output microstructured waveguide both consist of
a base material and regularly arranged air holes, and the cores of
the input microstructured waveguide and the output microstructured
waveguide both consist of the base material.
2. The terahertz waveguide according to claim 1, wherein the input
segment comprises an input tapered waveguide and an input straight
waveguide, a small end of the input tapered waveguide is connected
with a first end of the input straight waveguide, and a second end
of the input straight waveguide is connected with the first end of
the core of the input microstructured waveguide; and the output
waveguide is a tapered waveguide, and a small end of the output
waveguide is connected with the Second end of the core of the
output microstructured waveguide.
3. The terahertz waveguide according to claim 2, wherein
cross-sections of the input tapered waveguide, the input straight
waveguide-, the input microstructured waveguide, the output
microstructured waveguide, and the output waveguide are of circular
symmetry, the input tapered waveguide, the input straight waveguide
and the input microstructured waveguide are coaxial, the output
microstructured waveguide and the output waveguide are coaxial; the
small end of the output waveguide has a diameter d.sub.ts2, the
core of the output microstructured waveguide has a diameter
d.sub.m2, the small end of the input tapered waveguide has a
diameter d.sub.ts1, the input straight waveguide has a diameter
d.sub.z, and the core of the input microstructured waveguide has a
diameter d.sub.m1, wherein
d.sub.m2>d.sub.ts2>d.sub.m1>d.sub.ts1=d.sub.z>d.sub.c.
4. The terahertz waveguide according to claim 3, wherein the input
straight waveguide has a numerical aperture NA.sub.0, the input
microstructured waveguide has a numerical aperture NA.sub.1, the
sub-wavelength waveguide has a numerical aperture NA.sub.2, the
output microstructured waveguide has a numerical aperture NA.sub.3,
the small end of the output waveguide has a numerical aperture
NA.sub.4, the input microstructured waveguide has a mode field
diameter W.sub.1, and the output microstructured waveguide has a
mode field diameter W.sub.3, wherein
NA.sub.4d.sub.ts2>NA.sub.0d.sub.z>NA.sub.1W.sub.1>NA.sub-
.2d.sub.c>NA.sub.3W.sub.3.
5. The terahertz waveguide according to claim 4, wherein the
following relationships are satisfied:
NA.sub.0d.sub.z=k.sub.1NA.sub.1W.sub.1,
NA.sub.1W.sub.1=k.sub.2NA.sub.2d.sub.c,
NA.sub.2d.sub.c=k.sub.3NA.sub.3W.sub.3,
NA.sub.4d.sub.ts2=k.sub.4NA.sub.3W.sub.3, wherein k.sub.1, k.sub.2,
k.sub.3, and k.sub.4 are coefficients, k.sub.1 ranges from 1.5 to
4, k.sub.2 ranges from 1 to 2, k.sub.3 ranges from 1 to 2, and
k.sub.4 ranges from 10 to 20, d.sub.z denotes the diameter of the
input straight waveguide, and de denotes the diameter of the
sub-wavelength waveguide.
6. The terahertz waveguide according to claim 1, wherein the
transmission segment comprises at least two sub-wavelength
waveguides arranged in parallel, one end of at least one of the at
least two sub-wavelength waveguides is connected with the core of
the input microstructured waveguide, and one end of at least one of
the at least two sub-wavelength waveguides is connected with the
core of the output microstructured waveguide.
7. The terahertz waveguide according to claim 6, wherein the
transmission segment comprises two sub-wavelength waveguides
arranged in parallel, one end of one of the two sub-wavelength
waveguides is connected with the core of the input microstructured
waveguide, and one end of the other one of the two sub-wavelength
waveguides is connected with the core of the output microstructured
waveguide.
8. The terahertz waveguide according to claim 6, wherein the
transmission segment comprises three sub-wavelength waveguides
arranged in parallel, one end of one of the three sub-wavelength
waveguides located in a middle position is connected with the core
of the input microstructured waveguide, and one end of each of the
other two of the three sub-wavelength waveguides is connected with
a respective one of two cores of the output microstructured
waveguide-.
9. The terahertz waveguide according to claim 1, wherein the input
microstructured waveguide comprises more than 1 layer of the
regularly arranged air holes, and the output microstructured
waveguide comprises no less than 1 layer of the regularly arranged
air holes; when the input microstructured waveguide or the output
microstructured waveguide comprises no less than 2 layers of the
regularly arranged air holes, a diameter of the regularly arranged
air holes in each of the no less than 2 layers increases gradually
in a radially outward direction from a corresponding core, and the
diameter of the regularly arranged air holes ranges from
.lamda..sub.0/20 to 3.lamda..sub.0.
10. The terahertz waveguide according to claim 1, wherein the
regularly arranged air holes in the input microstructured waveguide
or the output microstructured waveguide are arranged in regular
triangle-shaped grids or on circumferences of circles with a core
center of a corresponding microstructured waveguide as a center of
the circles.
11. The terahertz waveguide according to claim 9, wherein the
regularly arranged air holes in the input microstructured waveguide
or the output microstructured waveguide are arranged in regular
triangle-shaped grids or on circumferences of circles with a core
center of a corresponding microstructured waveguide as a center of
the circles.
Description
CROSS REFERENCE TO THE RELATED APPLICATIONS
[0001] This application is the national phase entry of
International Application No. PCT/CN2019/091643, filed on Jun. 18,
2019, which is based upon and claims priority to Chinese Patent
Application No. 201910466568.5, filed on May 31, 2019, the entire
contents of which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to the terahertz field, in
particular to a terahertz waveguide.
BACKGROUND
[0003] Terahertz waves have shown the special characteristics of
low energy, water absorption, strong penetration and so on, so it
has very important applications in medical imaging, chemistry,
biology, communication and other fields. Because the terahertz
signal is greatly affected by water vapor absorption and
atmospheric scattering, the attenuation coefficient of terahertz
waves in free space is so large that it cannot be transmitted over
a long distance. Therefore, to promote the development of terahertz
technology, the research on terahertz waveguide is of great
significance.
[0004] Some researchers have proposed a sapphire fiber with a
diameter of 150-325 .mu.m, which can realize low loss single-mode
transmission of terahertz wave. On this basis, some researchers
proposed a sub-wavelength terahertz solid core fiber. The diameter
of the fiber core is about 200 .mu.m, the air outside the core is
worked as the fiber cladding, and the core is made of polyethylene
(PE). The loss near 0.3 THz is less than 0.01 cm.sup.1. Due to the
small cross-section of the core, the coupling efficiency between
the fiber core and the terahertz source is only about 20%.
[0005] The main disadvantage of sub-wavelength solid core optical
fiber is that it needs special support components in the
application process, and the system stability is poor. In addition,
the fiber core is directly exposed to the air, and its transmission
performance is affected by the external environment and is easily
disturbed by the surrounding signals. Finally, due to the small
cross-section size of the core, the coupling between the core and
the terahertz source is also difficult.
SUMMARY
[0006] In view of the deficiencies in the prior art, the present
invention provides a terahertz waveguide, which solves the problem
that the conventional sub-wavelength waveguide needs mechanical
support in the whole transmission direction, ensures the minimum
influence of the structure at both ends on the transmission of
internal terahertz wave in the main transmission section, and
realizes the isolation between the transmission waveguide and the
environment.
[0007] The present invention realizes the above technical purpose
through the following technical means.
[0008] Herein presents a terahertz waveguide, wherein the waveguide
includes an input segment, a transmission segment and an output
segment, which are listed as follows:
[0009] An input segment, wherein the input segment includes an
input waveguide (1), and an input microstructured waveguide,
wherein one end of the input waveguide is connected with one end of
the core of the input microstructured waveguide;
[0010] A transmission segment, wherein the transmission segment
includes at least a sub-wavelength waveguide (3), an air cladding
surrounding the sub-wavelength waveguide (3) and a solid outer
cladding (6) surrounding the air cladding, wherein the other end of
the input microstructured waveguide is connected with one end of
the sub-wavelength waveguide, wherein the other end of the
sub-wavelength waveguide is connected with the core of the output
microstructured waveguide, wherein one end of the solid outer
cladding are connected with the cladding of the input
microstructured waveguide;
[0011] An output segment, wherein the output segment includes an
output microstructured waveguide (4) and an output waveguide (5),
wherein the other end of the sub-wavelength waveguide (3) is
connected with the end of the core of the output microstructured
waveguide (4), the other end of the outer cladding (6) is connected
with one end of the cladding of the output microstructured
waveguide (4), the other end of the core of the output
microstructured waveguide (4) is connected with one end of the
output waveguide (5).
[0012] The diameter of the sub-wavelength waveguide (3) d.sub.c
satisfies dcD<.lamda..sub.0, where .lamda..sub.0 is the
operating wavelength, wherein the cladding of the input and output
microstructured waveguides consists of base material and regularly
arranged air holes, and the core of the microstructured waveguides
consists of base material.
[0013] Preferably, the input segment includes an input tapered
waveguide (1-1) and an input straight waveguide (1-2), the small
end of the input tapered waveguide (1-1) is connected with one end
of the input straight waveguide (1-2), and the other end of the
input straight waveguide is connected with one end of the core of
the input microstructured waveguide (2), wherein the output
waveguide (5) is a tapered waveguide, and the small end of the
output waveguide (5) is connected with the other end of the core of
the microstructured waveguide.
[0014] Preferably, the cross-sections of the input tapered
waveguide (1-1), the input straight waveguide (1-2), the input
microstructured waveguide (2), the output microstructured waveguide
(4), and the output tapered waveguide (5) are of circular symmetry,
wherein the input tapered waveguide (1-1), the input straight
waveguide (1-2) and the input microstructured waveguide (2) are
coaxial, wherein the output microstructured waveguide (4) and the
output waveguide (5) are coaxial, wherein the waveguide parameters
should meet the condition of
d.sub.m2>d.sub.ts2>d.sub.m1>d.sub.ts1=d.sub.z>d.sub.c,
where d.sub.ts2, d.sub.m2, d.sub.ts1, d.sub.z, d.sub.m1 denote
respectively the diameter of the small end of the output waveguide
(5), the core diameter of the output microstructured waveguide (4),
the diameter of the small end of the input tapered waveguide (1-1),
the diameter of the input straight waveguide (1-2), and the core
diameter of the input microstructured waveguide (2).
[0015] Preferably, the numerical aperture, the mode field diameter
and the waveguide diameter of the segments should meet the
conditions of
NA.sub.4d.sub.ts2>NA.sub.0d.sub.z>NA.sub.1W.sub.1>NA.sub.2d.sub.-
c>NA.sub.3W.sub.3, where NA.sub.0, NA.sub.1, NA.sub.2, NA.sub.3
and NA.sub.4 denote the numerical apertures of the input straight
waveguide (1-2), the input microstructured waveguide (2), the
sub-wavelength waveguide (3), the output microstructured waveguide
(4) and the small end of the output tapered waveguide (5),
respectively, wherein W.sub.1 and W.sub.3 are the mode field
diameters of the input and output microstructured waveguide,
respectively.
[0016] Preferably, the numerical apertures, the mode field
diameters and the waveguide diameters of the segments should meet
the conditions of
NA.sub.0d.sub.z=k.sub.1NA.sub.1W.sub.1
NA.sub.1W.sub.1=k.sub.2NA.sub.2d.sub.c
NA.sub.2d.sub.c=k.sub.3NA.sub.3W.sub.3
NA.sub.4d.sub.ts2=k.sub.4NA.sub.3W.sub.3
[0017] wherein k.sub.1, k.sub.2, k.sub.3, k.sub.4 are coefficients,
the range of k.sub.1 is 1.5 to 4, the range of k.sub.2 is 1 to 2,
the range of k.sub.3 is 1 to 2, and the range of k.sub.4 is 10 to
20, wherein d.sub.z denotes the diameter of the input straight
waveguide (1-2), d.sub.c denotes the diameter of the sub-wavelength
waveguide (3).
[0018] Preferably, the number of sub-wavelength waveguides at the
transmission segment should meet the condition of n2, wherein the
sub-wavelength waveguides (3) are arranged in parallel and at least
one end of a sub-wavelength waveguide (3) is connected with the
core of the input microstructured waveguide (2), and at least one
end of a sub-wavelength waveguide (3) is connected with the core of
the output microstructured waveguide (4).
[0019] Preferably, the transmission segment is composed of two
sub-wavelength waveguides (3) arranged in parallel, and one end of
a sub-wavelength waveguide (3) is connected with the core of the
input microstructured waveguide (2), and one end of the other
sub-wavelength waveguide (3) is connected with the core of the
output microstructured waveguide (4).
[0020] Preferably, the transmission segment is composed of three
sub-wavelength waveguides (3) arranged in parallel, and one end of
the central sub-wavelength waveguide (3) is connected with the core
of the input microstructured waveguide (2), and one end of the two
sub-wavelength waveguides (3) on the sides are connected with one
of the two cores of the output microstructured waveguide (4),
respectively.
[0021] Preferably, the number of air hole rings N should meet the
condition of N>1 for the input microstructured waveguide (2) and
the number of air hole rings N should meet the condition of N>=1
for the output microstructured waveguide (4), wherein for the
microstructure waveguide composed of least two rings of air holes,
the diameter of air holes in each ring should increase gradually
along radial direction, and the diameters of all air holes should
be ranged from .lamda..sub.0/20-3 .lamda..sub.0.
[0022] Preferably, the air holes are arranged in regular triangular
meshes or in a circle ring for both the input microstructured
waveguide and the output microstructured waveguide, the centers of
the circle rings are the corresponding core centers of the
microstructured waveguides.
[0023] The beneficial effects of the present invention are as
follows:
[0024] 1) The input and output microstructured waveguides are
respectively arranged at both ends of the transmission end, which
overcomes the shortcoming that conventional sub-wavelength
waveguides need mechanical support in the whole transmission
direction, thus optimizing the mode field distribution, reducing
the confinement loss, etc. At the same time, the outer cladding at
the transmission segment realizes the isolation of the
sub-wavelength waveguide from the external environment and avoids
the disturbance of the environment. In the sensor application, the
external medium can be introduced to realize the special sensing
function by cutting a part of the outer cladding.
[0025] 2) The input segment is composed of an input tapered
waveguide, a input straight waveguide and an input microstructured
waveguide, so as to realize the conversion of a large input mode
field into a matched mode field of the sub-wavelength waveguide, so
as to realize low loss transmission.
[0026] 3) The output waveguide of the present invention is a
tapered waveguide, and the output segment is composed of an output
waveguide and an output microstructured waveguide, which can
effectively couple the mode field transmitted by the sub-wavelength
waveguide.
[0027] 4) The structure of the present invention can easily form a
multi waveguide transmission structure to realize mode coupling,
conversion and other functions between the waveguides. Moreover,
the present invention can form a cascade structure to construct
more complex functional devices. As the input and output segments
are both solid core waveguide structures, as long as the connection
segments match, different sizes of sub-wavelength waveguides can
realize low loss connection.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a structural diagram of an embodiment of a
terahertz waveguide of the present invention.
[0029] FIG. 2 is a schematic diagram of the input segment of the
terahertz waveguide shown in FIG. 1.
[0030] FIG. 3 is a schematic diagram of the cross-section of the
input microstructured waveguide shown in FIG. 1.
[0031] FIG. 4 is a schematic diagram of the output segment of the
terahertz waveguide shown in FIG. 1.
[0032] FIG. 5 is a cross-section diagram of the output
microstructured waveguide shown in FIG. 4.
[0033] FIG. 6 is a cross-sectional view of the transmission segment
of the terahertz waveguide shown in FIG. 1.
[0034] FIGS. 7A-7D are the k-value diagrams of the terahertz
waveguide shown in FIG. 1, wherein FIG. 7A is the scatter diagram
of k.sub.1 varying with the diameter d.sub.z of the input straight
waveguide, FIG. 7B is the scatter diagram of k.sub.2 varying with
the diameter d.sub.in-3 of the innermost ring air holes of the
input microstructured waveguide, FIG. 7C is the scatter diagram of
k.sub.3 varying with the diameter de of the subwavelength
waveguide, and FIG. 7D is the scatter diagram of k.sub.4 varying
with the diameter d.sub.out of the air holes in the output
microstructured waveguide.
[0035] FIGS. 8A-8C are the mode field diagrams of the terahertz
waveguide shown in FIG. 1, wherein FIG. 8A is the fundamental mode
diagram of the input straight waveguide, FIG. 8B is the fundamental
mode diagram of the input microstructured waveguide, and FIG. 8C is
the fundamental mode diagram of the waveguide at the transmission
segment.
[0036] FIG. 9 is a cross-sectional view of a terahertz waveguide
with two sub-wavelength waveguides of a terahertz waveguide
according to an embodiment of the present invention.
[0037] FIG. 10 is a sectional view of a terahertz waveguide with
three sub-wavelength waveguides according to an embodiment of the
present invention.
[0038] FIG. 11 shows the transmission efficiency spectrum of a
terahertz waveguide according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0039] Embodiments of the present invention are described in detail
below, examples of which are shown in the accompanying drawings, in
which the same or similar labels from beginning to end indicate the
same or similar elements or elements with the same or similar
functions. The embodiments described below by reference to the
accompanying drawings are illustrative and are intended to be used
for the interpretation of the present invention and cannot be
understood as a limitation of the present invention.
[0040] In the description of the present invention, it is to be
understood that the terms "center", "longitudinal", "transverse",
"length", "width", "thickness", "up", "down", "axial", "radial",
"vertical", "horizontal", "inner", "outer" and the like refer to
the orientation or position relationship shown in the attached
drawings are used for the description of the present invention and
the simplified description, rather than indicating or implying that
the device or element in question must have a specific orientation,
be constructed and operated in a specific orientation, and
therefore cannot be understood as a limitation of the present
invention. In addition, the terms "first" and "second" are used
only for the purpose of description and cannot be understood as
indicating or implying relative importance or implicitly indicating
the number of technical features indicated. Thus, the features
defined as "first" and "second" may explicitly or implicitly
include one or more of the features. In the description of the
present invention, "multiple" means two or more, unless otherwise
specifically defined.
[0041] In this invention, unless otherwise specified and limited,
the terms "installation", "interconnection", "connection",
"fixation" and other terms shall be understood in a broad sense.
For example, it can be fixed connection, detachable connection, or
integrated connection; it can be mechanical connection or
electrical connection; it can be direct connection or indirect
connection through intermediate medium, it can be the internal
connection of two components. For the technicians in the field, the
specific meaning of the above terms in the present invention can be
understood according to the specific situation.
[0042] An embodiment of the terahertz waveguide of the present
invention is described in detail with reference to the attached
drawings.
[0043] Referring to FIGS. 1 to 9, a terahertz waveguide according
to an embodiment of the present invention includes an input
segment, a transmission segment and an output segment.
[0044] Specifically, as shown in FIG. 1, the input includes an
input waveguide 1 and an input microstructured waveguide 2, the
transmission segment includes a sub-wavelength waveguide 3, an air
cladding surrounding the sub-wavelength waveguide and an outer
cladding 6 surrounding the air layer, and the output segment
includes an output microstructured waveguide 4 and an output
waveguide 5. One end of the input waveguide 1 is connected with one
end of the core of the input microstructured waveguide 2, the other
end of the core of the input microstructured waveguide 2 is
connected with one end of the sub-wavelength waveguide 3, one end
of the outer cladding 6 is connected with the cladding of the input
microstructured waveguide 2, the other end of the outer cladding 6
is connected with the cladding of the output microstructured
waveguide 4, and the other end of the sub-wavelength waveguide 3 is
connected with one end of the core of the output microstructured
waveguide 4. The other end of the core of the output microstructure
waveguide 4 is connected with one end of the output end waveguide
5.
[0045] The diameter d.sub.c of the sub-wavelength waveguide 3
satisfies d.sub.c<.lamda..sub.0, where .lamda..sub.0 is the
working wavelength. Sub-wavelength waveguide 3 can reduce the
transmission ratio of terahertz wave in the waveguide, thus
reducing the material absorption loss. The claddings of the input
microstructured waveguide 2 and the output microstructural
waveguide 4 are composed of base materials and regularly arranged
air holes, and the fiber cores of the input microstructured
waveguide 2 and the output microstructured waveguide 4 are composed
of base materials.
[0046] Therefore, the terahertz waveguide according to the
embodiment of the present invention, by connecting the input and
output microstructured waveguides at the two ends of the
transmission segment, the sub-wavelength waveguide 3 and the outer
cladding 6 are supported mechanically, which overcomes the
disadvantage that the conventional sub-wavelength waveguide needs
mechanical support in the whole transmission direction.
Simultaneously, as shown in FIG. 6, the outer cladding of the
transmission segment can isolate the sub-wavelength waveguide 3
from the external environment, so that the terahertz wave can be
transmitted in the sub-wavelength waveguide 3 and the air cladding,
thus avoiding the disturbance of the environment. The terahertz
waveguide of the embodiment of the present invention does not need
support elements in the longitudinal direction except that
microstructured waveguides are introduced to support the
sub-wavelength waveguide at both ends, so as to effectively reduce
the transmission loss of the waveguide because confinement loss
induced by the support elements is eliminated. In addition, the
input microstructured waveguide 2 and the output microstructural
waveguide 4 can also effectively convert the mode field of
terahertz waveguide mode, so that the terahertz wave in the input
segment can effectively couple into the sub-wavelength waveguide 3,
and the terahertz wave output from the sub-wavelength waveguide 3
can be effectively coupled to the output segment.
[0047] If it is necessary to connect two terahertz waveguides
according to the embodiment of the present invention, for example,
to connect the output segment of the first terahertz waveguide with
the input segment of the second terahertz waveguide, it is only
necessary to design the structure of the two terahertz waveguides
so that the output end face size of the output waveguide of the
first terahertz waveguide is the same as the input end face size of
the input waveguide of the second terahertz waveguide, then low
loss connection between them can be realized. Therefore, the
terahertz waveguide according to the embodiment of the present
invention can easily form cascade structure, thus forming more
complex waveguide devices and realizing more complex functions.
[0048] Preferably, in this embodiment, the input segment includes
an input tapered waveguide 1-1 and an input straight waveguide 1-2,
the narrow end of the input tapered waveguide 1-1 is connected with
one end of the input straight waveguide 1-2, and the other end of
the input straight waveguide 1-2 is connected with one end of the
core of the input microstructured waveguide 2. The output waveguide
5 is a tapered waveguide, and the narrow end of the output
waveguide 5 is connected with the other end of the core of the
output microstructured waveguide 4.
[0049] The input segment is composed of tapered waveguide 1-1,
input straight waveguide 1-2 and input microstructured waveguide 2,
which can convert large input mode field into matched mode field of
the sub-wavelength waveguide, so as to realize low loss
transmission. Considering the matching between the spot diameter
and the waveguide aperture, the spot size of different terahertz
wave sources is different. The wide end of the tapered waveguide
1-1 at the input end can be flexibly adjusted to meet different
wave sources. On the other hand, the straight waveguide 1-2 can
play a transitional role in adjusting the waveform so that the wave
can be better coupled into the input microstructured waveguide
4.
[0050] The output segment is composed of an output tapered
waveguide 5 and an output microstructured waveguide 4, which can
effectively couple the mode field transmitted by sub-wavelength
waveguide, and expand the mode field diameter of the waveform to a
certain extent to adapt to the input port size of a terahertz
detector. The input tapered waveguide 1-1, the input straight
waveguide 1-2 and the output waveguide 5 all have large
cross-sectional dimensions, which can effectively reduce the energy
ratio of the terahertz wave in the air, and can effectively connect
with terahertz sources.
[0051] Preferably, in this embodiment, the cross-sections of the
input tapered waveguide 1-1, the input straight waveguide 1-2, the
input microstructured waveguide 2, the output microstructural
waveguide 4 and the output waveguide 5 are circular, in addition,
the input tapered waveguide 1-1, the input straight waveguide 1-2
and the input microstructured waveguide 2 are coaxial, whereas the
output microstructured waveguide 4 and the output waveguide 5 are
coaxial.
[0052] The diameters of the waveguides should meet the condition of
d.sub.m2>d.sub.ts2>d.sub.m1>>d.sub.rs1=d.sub.z>d.sub.c,
where d.sub.ts2 is the narrow end diameter of the output waveguide
5, d.sub.m2 is the core diameter of the output microstructured
waveguide 4, d.sub.ts1 is the narrow end diameter of the input
tapered waveguide 1-1, d.sub.z is the diameter of the input
straight waveguide 1-2, and d.sub.m1 is the core diameter of the
input microstructured waveguide 2. The core radius of the
microstructured waveguide 2 or 4 is defined as the distance between
the center of the core and the center of the innermost air hole
minus the radius of the innermost air hole, wherein the core
diameter of the microstructured waveguide equals twice of the core
radius of the microstructured waveguide.
[0053] The parameters are set based on the actual conditions of
waveguide fabrication and the stability of waveguide mechanical
structure and preventing the air holes from being covered or
damaged.
[0054] Preferably, the length of the input tapered waveguide is set
as 1/2 .lamda..sub.0-5 .lamda..sub.0, the length of the input
straight waveguide is set as 1/2 .lamda..sub.0-5 .lamda..sub.0, the
length of the input microstructured waveguide is set as 1/2
.lamda..sub.0-5 .lamda..sub.0, and the length of the transmission
segment is set as 5-100 cm, which can be set based on the required
length and absorption loss. The length of output microstructured
waveguide is set as .lamda..sub.0-30 .lamda..sub.0, and the length
of output tapered waveguide is set as .lamda..sub.0-30
.lamda..sub.0.
[0055] Because the input microstructured waveguide 2 and the output
microstructured waveguide 4 are composed of base material and air
holes arranged on the base material, and their numerical aperture
is smaller than that of the sub-wavelength waveguide 3. Therefore,
the core diameter of output microstructured waveguide 4 should be
larger than that of the input microstructured waveguide 2, so that
the terahertz wave output from the sub-wavelength waveguide 3 can
be effectively coupled into the output microstructured waveguide
4.
[0056] Based on the formula of fundamental mode diameter
d = 2 .times. 2 .pi. .times. .lamda. 0 NA , ##EQU00001##
where NA is the maximum theoretical numerical aperture of
single-mode waveguide, and the relationship between the power of
fundamental mode core and the diameter of fiber core obtained from
experiments, it is found that the numerical apertures of the
waveguides should meet the conditions of
NA.sub.4d.sub.ts2>NA.sub.0d.sub.z>NA.sub.1W.sub.1>NA.sub.2d.sub.-
c>NA.sub.3W.sub.3, where NA.sub.0 is the numerical aperture of
the input straight waveguide 1-2, NA.sub.1 is the numerical
aperture of the input microstructured waveguide 2, NA.sub.2 is the
numerical aperture of the sub-wavelength waveguide 3, NA.sub.3 is
the numerical aperture of the output microstructured waveguide 4,
NA.sub.4 is the numerical aperture of the narrow end of the output
waveguide 5, W.sub.1 is the mode field diameter of the input
microstructured waveguide 2, and the mode field diameter of the
output microstructured waveguide 5 is W.sub.3. The numerical
relationship is obtained by simulation. The mode field diameter
decreases gradually from the large diameter end of the input
tapered waveguide 1-1, so that most of the terahertz waves can be
effectively coupled into the sub-wavelength waveguide 3, when the
terahertz waves reach the output waveguide 5, the mode field will
expand, and the mode field diameter increases, so that the wave can
be effectively coupled into the corresponding receiving
equipment.
[0057] Furthermore, the numerical apertures should satisfy the
following relations NA.sub.0d.sub.z=k.sub.1NA.sub.1W.sub.1,
NA.sub.1W.sub.1=k.sub.2NA.sub.2d.sub.c,
NA.sub.2d.sub.c=k.sub.3NA.sub.3W.sub.3,
NA.sub.4d.sub.ts2=k.sub.4NA.sub.3W.sub.3.
[0058] Among them, d.sub.z is the diameter of the input straight
waveguide 1-2, d.sub.c is the diameter of sub wavelength waveguide
3, and k.sub.1, k.sub.2, k.sub.3, k.sub.4 are proportional
coefficients. As can be seen from FIGS. 7A-7D, the value range of
k.sub.1 is 1.5-4, the value range of k.sub.2 is 1-2, the value
range of k.sub.3 is 1-2, and the value range of k.sub.4 is 10-20.
When the above relationship is met, terahertz wave can be
transmitted efficiently.
[0059] It can be seen from FIGS. 8A-8C that there are large
differences in the mode field distribution and mode field size
between the input straight waveguide 1-2 and the sub-wavelength
waveguide 3. The cladding of the input straight waveguide 1-2 is
air, while the core of the input microstructured waveguide 2 is
surrounded by air holes. Therefore, the refractive index
distribution of the two waveguides is similar, which can
effectively reduce the connection loss between the two waveguides.
Although the core diameter of the input microstructured waveguide 4
is larger than that of the sub-wavelength waveguide 3, its
numerical aperture is smaller than that of the sub-wavelength
waveguide 3, therefore, low loss connection can still be
realized.
[0060] Preferably, in the present embodiment, the ring number of
air holes in the input microstructured waveguide 2 is greater than
1, and the ring number of air holes in the output microstructured
waveguide 4 is not less than 1, which is usually set as 1-8. For
the condition of the ring number of air holes is not less than 2
either in the input microstructured waveguide 2 or in the output
microstructured waveguide 4, the diameter of air holes in each ring
should gradually increase along radial direction, that is, the
inner air holes have smaller diameter than the outer air holes. The
reason is that the lower the equivalent cladding refractive index
is, the lower the absorption loss is, and the higher the efficiency
of terahertz wave transmission. The diameter range of air hole
should be in the range of .lamda..sub.0/20-3 .lamda..sub.0, which
can ensure the mechanical stability of the waveguide structure.
[0061] Furthermore, the arrangement of air holes in the input
microstructured waveguide 2 and the output microstructured
waveguide 4 can be selected according to the waveguide function to
realize the waveguide beam splitter, coupler, etc. In general, the
core is a solid polymer material, and there can be multiple air
holes outside the core. Furthermore, as shown in FIG. 3, the air
holes in the input microstructured waveguide 2 are arranged in the
regular triangular grid. It should be noted that the air holes in
the input microstructured waveguide 2 in the present embodiment are
not limited to the regular triangular grid, but can also be
arranged on the circumference with the core center as the center of
the circumference. The arrangement requirement of air holes in the
output microstructured waveguide 4 is also the same.
[0062] Preferably, in the present embodiment, the transmission
segment includes at least two sub-wavelength waveguides 3 arranged
in parallel, wherein one end of at least one sub-wavelength
waveguide 3 is connected with the core of the input microstructured
waveguide 2, and one end of at least one sub-wavelength waveguide 3
is connected with the core of the output microstructure waveguide
4.
[0063] Further, as shown in FIG. 9, the transmission segment
includes two sub-wavelength waveguides 3 arranged in parallel, one
end of one sub-wavelength waveguide 3 is connected with the core of
the input microstructured waveguide 2, and one end of the other
sub-wavelength waveguide 3 is connected with the core of the output
microstructured waveguide 4. This structure can couple the input
terahertz wave to another port and output it. By using the
characteristics of coupling length dependence on wavelength, this
structure can realize filtering and other functions.
[0064] Further, as shown in FIG. 10, the transmission segment
includes three sub-wavelength waveguides 3 arranged in parallel,
one end of the sub-wavelength waveguide 3 at the middle position is
connected with the core of the input microstructured waveguide 2,
and one end of the other two sub-wavelength waveguides 3 is
respectively connected with the two cores of the output
microstructured waveguide 4. This structure can divide the input
terahertz wave into two sub-wavelength waveguides and couple them
out effectively, so as to realize the function of uniform beam
splitting.
[0065] Preferably, in the present embodiment, an example of the
waveguide structure is shown in FIG. 1. The diameter of the large
end of the input tapered waveguide 1-1 is 710 p m, the diameter of
the narrow end is 250 .mu.m and the length is 1000 .mu.m. The
diameter and length of the input straight waveguide 1-2 are 250
.mu.m and 1000 .mu.m respectively. For this embodiment, the value
of k.sub.1 is 3.11. The incident wavelength is 270 .mu.m, the
material is polyethylene, which has low terahertz absorption loss,
and the refractive index is 1.53.
[0066] There are three layers of air holes arranged outside the
core of the input microstructured waveguide 2. The air holes are
arranged in a regular triangular grid. The period P.sub.1 of the
air holes is 200 .mu.m. The diameter of the first air hole ring is
112 .mu.m, the diameter of the air holes in the second ring is 126
.mu.m and the diameter of the air holes in the third ring is 176
.mu.m. The length of the input microstructured waveguide 2 is 1400
.mu.m, the total diameter is 1680 .mu.m, and the value of K.sub.2
is 1.18.
[0067] The sub-wavelength waveguide 3 is a solid core polyethylene
waveguide with a diameter of 70 .mu.m, and the length of the main
waveguide can be flexibly selected. In this embodiment, the length
is set as 5 cm. The outer diameter D.sub.2 is 1490 .mu.m, the inner
diameter D.sub.1 is 1300 .mu.m, the cladding thickness D.sub.c is
190 .mu.m, and k.sub.3 is 1.17.
[0068] The output microstructured waveguide 4 has a ring of air
holes arranged in a regular triangular grid. The period P.sub.2 of
the air hole is 240 .mu.m. The distance between the center of the
air holes and the center of the cross-section of the waveguide 3 in
the same horizontal plane is 720 .mu.m. The diameter of the air
holes is 120 .mu.m.
[0069] The narrow end of the output waveguide 5 is connected with
the output microstructured waveguide 4 based on the central axes of
the two waveguides. The diameter of the narrow end is 1140 .mu.m.
The terahertz wave is emitted from the wide end of the output
waveguide 5. The diameter of the wide end is 1440 .mu.m. It can be
effectively connected with equipment such as terahertz time domain
spectroscopy. The value of k.sub.4 is 14.12.
[0070] According to the calculation, the embodiment should meet the
condition of
NA.sub.4d.sub.ts2>NA.sub.0d.sub.z>NA.sub.1W.sub.1>NA.sub.2d.sub-
.c>NA.sub.3W.sub.3.
[0071] The transmission theory of the above waveguide components,
whether it is the input microstructured waveguide 2, the output
microstructured waveguide 4 or the sub-wavelength waveguide 3, is
based on total reflection theory. Considering the structural loss,
as shown in FIG. 11, the final fundamental mode output efficiency
can reach about 80%.
[0072] In the description of this specification, a description of
the reference terms "one embodiment," "some embodiments,"
"examples," "specific examples," or "some examples" and the like
means that specific features, structures, materials, or features
described in connection with the embodiments or examples are
included in at least one embodiment or example of the present
invention. In this specification, a schematic description of the
above terms does not necessarily refer to the same embodiment or
example. Moreover, the specific features, structures, materials or
features described may be combined in an appropriate manner in any
one or more embodiments or examples.
[0073] Although the embodiments of the present invention have been
shown and described above, it can be understood that the above
embodiments are illustrative and cannot be understood as a
limitation of the present invention. A technician in the art can
change, modify, replace and modify the above-mentioned embodiments
within the scope of the present invention without departing from
the principle and purpose of the present invention.
* * * * *