U.S. patent application number 16/145977 was filed with the patent office on 2020-04-02 for channel waveguides with bend compensation for low-loss optical transmission.
The applicant listed for this patent is CORNING RESEARCH & DEVELOPMENT CORPORATION. Invention is credited to Lars Martin Otfried Brusberg, Sergey Anatol'evich Kuchinsky, Rostislav Vatchev Roussev, Aramais Robert Zakharian.
Application Number | 20200106147 16/145977 |
Document ID | / |
Family ID | 1000004700255 |
Filed Date | 2020-04-02 |
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United States Patent
Application |
20200106147 |
Kind Code |
A1 |
Brusberg; Lars Martin Otfried ;
et al. |
April 2, 2020 |
CHANNEL WAVEGUIDES WITH BEND COMPENSATION FOR LOW-LOSS OPTICAL
TRANSMISSION
Abstract
The channel waveguides disclosed herein have bend compensation
in the form of at least one compensated bend section. The channel
waveguides are formed in a glass-based substrate having a
glass-based matrix. The channel waveguide has an waveguide IOX
region with a straight section and a bend section. The waveguide
IOX region at the bend section is superimposed with a quasi-linear
modifying IOX region to form a compensated bend IOX region that
defines the compensated bend section. The compensated bend section
has a reduced amount of optical loss as compared to if the
compensated bend section had a refractive index profile that was
the same as the straight section. Methods of forming the
compensated bend sections for the channel waveguides are also
disclosed.
Inventors: |
Brusberg; Lars Martin Otfried;
(Corning, NY) ; Kuchinsky; Sergey Anatol'evich;
(St. Petersburg, RU) ; Roussev; Rostislav Vatchev;
(Painted Post, NY) ; Zakharian; Aramais Robert;
(Painted Post, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CORNING RESEARCH & DEVELOPMENT CORPORATION |
Corning |
NY |
US |
|
|
Family ID: |
1000004700255 |
Appl. No.: |
16/145977 |
Filed: |
September 28, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01P 1/022 20130101;
G02B 2006/12038 20130101; G02B 6/1221 20130101 |
International
Class: |
H01P 1/02 20060101
H01P001/02; G02B 6/122 20060101 G02B006/122 |
Claims
1. A method of forming a channel waveguide with a compensated bend
section and a straight section comprising a straight-section
refractive index cross-sectional profile, the method comprising: a)
forming in a glass-based substrate a modifying ion-exchanged (IOX)
region using a first IOX process that uses a mask comprising a mask
opening that resides closest to a concave side of the compensated
bend section; b) forming in the glass-based substrate a waveguide
bend IOX region using a second IOX process designed to impart a
straight-section cross-sectional refractive index profile in the
absence of the modifying IOX region; and c) wherein the modifying
IOX region and the waveguide bend IOX region combine to form a
compensated waveguide bend IOX region that defines in the channel
waveguide the compensated bend section, which has an amount of
optical loss less than if the modifying IOX region were absent.
2. The method according to claim 1, further comprising performing
act a) prior to act b).
3. The method according to claim 1, wherein the glass-based
substrate comprises an aluminosilicate glass or a borosilicate
glass containing sodium, and wherein the first IOX process
comprises exchanging potassium ions (K.sup.+) for first sodium ions
(Na.sup.+) in the glass-based substrate, and wherein the second IOX
process comprises exchanging silver ions (Ag.sup.+) for additional
sodium ions (Na.sup.+) in the glass-based substrate.
4. The method according to claim 1, wherein the glass-based
substrate comprises an aluminosilicate glass or a borosilicate
glass containing sodium, and wherein the first IOX process
comprises exchanging potassium ions (K.sup.+) for first lithium
ions (Li.sup.+) in the glass-based substrate, and wherein the
second IOX process comprises exchanging silver ions (Ag+) for
additional lithium ions (Li.sup.+) in the glass-based
substrate.
5. The method according to claim 1, wherein: the mask opening has a
first arcuate opening with a first radius of curvature; the second
IOX process uses a second mask having a second arcuate opening with
a second radius of curvature; and wherein the first and second
arcuate openings have substantially the same center of curvature,
and wherein the first radius of curvature is less than the second
radius of curvature.
6. The method according to claim 1, wherein the compensated bend
section has a concave side and a convex side, and wherein modifying
IOX region has a higher refractive index on the concave side than
on the convex side.
7. (canceled)
8. The method according to claim 1, wherein the bend section has a
bend radius RB>1 mm and a width WW<100 .mu.m.
9. The method according to claim 8, wherein the bend radius RB is
in the range 5 mm.ltoreq.RB.ltoreq.30 mm and the width WW>5
.mu.m.
10. The method according to claim 1, wherein the modifying IOX
region has a refractive index contrast in the range from about
0.004 to about 0.006 at the waveguide bend IOX region.
11. The method according to claim 1, wherein a transition between
the compensated bend section and the straight waveguide section is
adiabatic.
12.-20. (canceled)
21. A channel waveguide formed by the process of claim 1.
22. A method of forming a channel waveguide with a compensated bend
section and a straight section comprising a straight-section
refractive index cross-sectional profile, the method comprising: a)
forming in a glass-based substrate a modifying ion-exchanged (IOX)
region using a first IOX process that uses a first mask having a
first arcuate opening with a first radius of curvature; b) forming
in the glass-based substrate a waveguide bend IOX region using a
second IOX process that uses a second mask having a second arcuate
opening with a second radius of curvature, the second IOX process
designed to impart a straight-section cross-sectional refractive
index profile in the absence of the modifying IOX region, wherein
the first and second arcuate openings have substantially the same
center of curvature, and the first radius of curvature is less than
the second radius of curvature; and c) wherein the modifying IOX
region and the waveguide bend IOX region combine to form a
compensated waveguide bend IOX region that defines in the channel
waveguide the compensated bend section, which has an amount of
optical loss less than if the modifying IOX region were absent.
23. The method according to claim 1, wherein the glass-based
substrate comprises an aluminosilicate glass or a borosilicate
glass containing sodium, and wherein the first IOX process
comprises exchanging potassium ions (K.sup.+) for first sodium ions
(Na.sup.+) in the glass-based substrate, and wherein the second IOX
process comprises exchanging silver ions (Ag.sup.+) for additional
sodium ions (Na.sup.+) in the glass-based substrate.
24. The method according to claim 1, wherein the glass-based
substrate comprises an aluminosilicate glass or a borosilicate
glass containing sodium, and wherein the first IOX process
comprises exchanging potassium ions (K.sup.+) for first lithium
ions (Li.sup.+) in the glass-based substrate, and wherein the
second IOX process comprises exchanging silver ions (Ag+) for
additional lithium ions (Li.sup.+) in the glass-based
substrate.
25. The method according to claim 1, wherein the compensated bend
section has a concave side and a convex side, and wherein the
modifying IOX region has a higher refractive index on the concave
side than on the convex side.
26. The method according to claim 1, wherein the first arcuate
opening resides closest to the concave side of the compensated bend
section.
27. The method according to claim 1, wherein the bend section has a
bend radius RB>1 mm and a width WW<100 .mu.m.
28. The method according to claim 8, wherein the bend radius RB is
in the range 5 mm.ltoreq.RB.ltoreq.30 mm and the width WW>5
.mu.m.
29. The method according to claim 1, wherein the modifying IOX
region has a refractive index contrast in the range from about
0.004 to about 0.006 at the waveguide bend IOX region.
30. The method according to claim 1, wherein a transition between
the compensated bend section and the straight waveguide section is
adiabatic.
Description
FIELD
[0001] The present disclosure generally relates to channel
waveguides, and in particular to channel waveguides with bend
compensation for low-loss optical transmission.
BACKGROUND
[0002] A photonic integrated device (also called a photonic
integrated circuit (PIC)) combines optical or both optical and
electronic components on a chip, in analogy to an electronic
integrated circuit. A typical photonic integrated device carries
and processes optical signals that travel in an array or network of
waveguides, which can be optically coupled to optical fibers to
carry the optical signal to and from the photonic integrated
device.
[0003] Certain types of photonic integrated devices make use of
planar lightwave circuits (PLCs) that support optical waveguides
("waveguides") used to communicate light between the device
components. A relatively high level of integration can be realized
by forming the waveguides in a thin glass-based substrate. The
efficient routing of the waveguide between device components
requires that at least some of the waveguides include one or more
bends. Unfortunately, such bends introduce loss of the optical
signal power. Since there are other sources of optical signal loss
(e.g., at optical interconnections, attenuation per distance
traveled in the optical fibers and waveguides, etc), it is
important to reduce optical signal loss wherever possible to avoid
having to amplify the optical signal or to increase detection
sensitivity.
SUMMARY
[0004] The disclosure is directed to methods for forming a channel
waveguide in a glass-based substrate for optical signal
transmission along with the channel waveguide. The method is
directed to forming a channel waveguide comprising a compensated
bend section and a straight-section comprising a straight-section
refractive index cross-sectional profile. The method comprising
forming in a glass-based substrate a quasi-linear modifying
ion-exchanged (IOX) region using a first IOX process, forming in
the glass-based substrate a waveguide bend IOX region using a
second IOX process designed to impart a straight-section
cross-sectional refractive index profile in the absence of the
quasi-linear modifying IOX region, and wherein the quasi-linear
modifying IOX region and the waveguide bend IOX region combine to
form a compensated waveguide bend IOX region that defines in the
channel waveguide the compensated bend section, which has an amount
of optical loss less than if the modifying IOX region were
absent.
[0005] The disclosure is also directed to a channel waveguide
comprising a glass-based substrate comprising a glass-matrix and a
surface, at least one ion-exchanged channel waveguide formed in the
glass-based matrix and comprising a compensated bend section and a
straight section, and wherein the straight section comprises a
waveguide IOX region comprising a first refractive index profile
and where the compensated bend section comprises the waveguide IOX
region comprising a bend section and superimposed with a
quasi-linear modifying IOX region that forms a compensated bend IOX
region defining a compensated bend section with a reduced amount of
optical loss as compared to if the compensated bend section
consisted only of the waveguide IOX region.
[0006] Additional features and advantages will be set forth in the
detailed description which follows, and, in part, will be apparent
to those skilled in the art from that description or recognized by
practicing the embodiments as described herein, including the
detailed description which follows the claims, as well as the
appended drawings.
[0007] It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary and are intended to provide an overview or framework to
understanding the nature and character of the claims. The
accompanying drawings are included to provide a further
understanding, and are incorporated in and constitute a part of
this specification. The drawings illustrate one or more
embodiments, and together with the description explain the
principles and operation of the various embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1A is a top-down view of an example PLC formed in a
glass-based substrate and that supports multiple channel waveguides
each having at least one bend and showing the global Cartesian
coordinates x, y, z;
[0009] FIG. 1B is a x-y cross-sectional view of the PLC of FIG.
1A;
[0010] FIG. 2 is a close-up top-down view of an example section of
a channel waveguide formed in the glass-based substrate, wherein
the section includes bend section and two straight portions.
[0011] FIG. 3 is a cross-sectional view in the local (x,y)
coordinates of an example waveguide IOX region for a straight
portion of a channel waveguide as formed in the matrix of the
glass-based substrate, with the straight-portion having a
refractive index profile n.sub.S(x,y) as shown by the refractive
index contours.
[0012] FIG. 4 is similar to FIG. 3 and shows an example of a linear
modifying IOX region with a linear modifying refractive index
profile n.sub.M(x,y).
[0013] FIG. 5 is a plot based on a computer-simulation that
combines a linear modifying IOX region such as shown in FIG. 4 with
a waveguide IOX region for a straight portion of a channel
waveguide such as shown in FIG. 3 to arrive at compensated
cross-sectional refractive index profile n.sub.C(x,y) for the bend
section of the waveguide.
[0014] FIG. 6A is an (x,y) cross-sectional view of an example
modifying IOX process that forms a quasi-linear modifying IOX
region in the matrix of the glass-based substrate;
[0015] FIG. 6B is a top-down view of a first mask and the
refractive index contours of the modifying IOX region formed using
the modifying IOX process of FIG. 6A.
[0016] FIG. 7 is a plot of the refractive index contrast dn(y)
versus distance y (.mu.m) into the glass-based substrate for both
transverse electric (TE) and tranverse magnetic (TM) polarizations
for an example K.sup.+-Na.sup.+ IOX process carried out in an
aluminosilicate substrate using a KNO.sub.3 salt bath at a
temperature of 380.degree. C. for a diffusion time of few
hours.
[0017] FIG. 8A is similar to FIG. 6A and is an (x,y)
cross-sectional view of an example second IOX process that forms a
waveguide bend in the quasi-linear modifying IOX region, with the
modifying IOX refractive index contours superimposed with the
waveguide IOX refractive index contours.
[0018] FIG. 8B is atop-down view of the refractive index contours
of the quasi-linear modifying IOX and the second mask used to form
the waveguide bend section using the second IOX process, with the
location of the first mask aperture shown for reference (dashed
outline).
[0019] FIG. 8C is similar to FIG. 2 and is top-down view of a
section of the channel waveguide after forming the compensated bend
section and showing the refractive index profiles at six local
(x,y) cross-sectional planes CX0 through CX5.
[0020] FIG. 8D shows the local straight cross-sectional refractive
index profile for the cross-sectional planes CX0 and CX5 taken in
the straight portions of the channel waveguide.
[0021] FIG. 8E shows the local compensated cross-sectional
refractive index profile n.sub.C(x,y) for the cross-sectional
planes CX1 through CX4 taken in the compensated bend section of the
channel waveguide.
[0022] FIG. 9A is a top-down view of an example PLC formed in a
glass-based substrate and showing an example of two different
channel waveguides each having compensated bend section, and also
showing the corresponding modifying IOX regions that run adjacent
and parallel to the corresponding bend sections of the channel
waveguides to define the compensated bend IOX region for each of
the compensated bend sections.
[0023] FIG. 9B is similar to FIG. 9A and shows an example
configuration for the modifying IOX regions that provide an
adiabatic transition between the compensated bend section and the
adjacent straight portions of the channel waveguides.
[0024] FIG. 9C is a top-down view of a portion of a PLC that
includes a PLC device in the form of an optical coupler and showing
example configurations modifying IOX regions used to form the
compensated bend IOX regions that define the compensated bend
sections of the channel waveguides.
[0025] FIG. 10A is a plot of optical transmission T versus the
ratio G/G.sub.0, where G.sub.0=n.sub.0/R and G=n.sub.0/(RB.+-.dRB)
for a bend radius of RB=7.5 mm and for dRB in the range from 0 to
(1.5)RB.
[0026] FIG. 10B is similar to FIG. 10A and plots the bend loss BL
(dB) versus G/G.sub.0.
[0027] FIGS. 11A and 11B are the same as FIGS. 10A and 10B
respectively, but for a bend radius RB=3.0 mm.
DETAILED DESCRIPTION
[0028] Reference will now be made in detail to the present
embodiments, examples of which are illustrated in the accompanying
drawings. Whenever possible, the same reference numerals are used
throughout the drawings to refer to the same or like parts.
[0029] Cartesian coordinates are used in some of the Figures for
reference and are not intended to be limiting as to direction or
orientation. The Cartesian coordinates (x,y,z) are general or
global coordinates for the glass-based substrate. The Cartesian
coordinates (x,y) are specific or local coordinates of the
waveguide, where a given localized (x,y) plane is perpendicular to
the centerline of the waveguide, which is curved in bend sections
of the waveguide and straight in straight sections for the
waveguide. Thus, the localized z-axis is co-linear with the
waveguide centerline, and localized functions f(x,y), such as
refractive index n(x,y), are for a given localized z-position along
the centerline or localized z-axis. Unless otherwise stated, a
given function f(x,y) is substantially constant along the localized
z-axis in a given waveguide section, e.g., the bend section or the
straight section.
[0030] For purposes of description herein, the terms "upper,"
"lower," "right," "left," "rear," "front," "vertical,"
"horizontal," "top," "bottom," "side," and derivatives thereof,
shall relate to the disclosure as oriented with respect to the
Cartesian coordinates in the corresponding Figure, unless stated
otherwise. However, it is to be understood that the disclosure may
assume various alternative orientations, except where expressly
specified to the contrary.
[0031] The term "mode" is short for "guided mode" or "waveguide
mode," which describes an allowed spatial distribution of light
that propagates in a waveguide, whether it be an optical fiber or
substrate-based waveguide. A mode can have a transverse electric
(TE) polarization or a transverse magnetic (TM) polarization. A
single mode waveguide supports only one TE and one TM mode. Modes
are identified by a mode number m, where m=0 is the fundamental
mode and m=1, 2, 3, . . . are higher-order modes.
[0032] The abbreviation "nm" stands for "nanometer," which is
1.times.10.sup.-9 meter.
[0033] The abbreviation ".mu.m" stands for "micron" or
"micrometer," which is 1.times.10.sup.-9 meter.
[0034] The term "quasi-linear" as used herein means either linear
or nearly linear to an acceptable approximation. Thus, the
quasi-linear refractive index term .DELTA.n.sub.q(x,y) introduced
and discussed below can be equal to or fall within an acceptable
range of the exact linear term .DELTA.n(x)=Gx, where G is a
constant refractive index gradient.
[0035] It is also to be understood that the specific devices and
processes illustrated in the attached drawings, and described in
the following specification, are simply exemplary embodiments of
the inventive concepts defined in the appended claims. Hence,
specific dimensions and other physical characteristics relating to
the embodiments disclosed herein are not to be considered as
limiting unless the claims expressly state otherwise. Additionally,
embodiments depicted in the Figures may not be to scale or may
incorporate features of more than one embodiment.
[0036] As used herein, the term "and/or," when used in a list of
two or more items, means that any one of the listed items can be
employed by itself, or any combination of two or more of the listed
items can be employed. For example, if a composition is described
as containing components A, B, and/or C, the composition can
contain A alone; B alone; C alone; A and B in combination; A and C
in combination; B and C in combination; or A, B, and C in
combination.
[0037] The term "comprises" as used herein, such as in the phrase
"A comprises B," is intended to include as a special case "A
consists of B."
[0038] Example PLC
[0039] FIG. 1A is a top-down view and FIG. 1B is an x-y
cross-sectional view of an example PLC 10 formed in a glass-based
substrate 20. The glass-based substrate 20 has a glass-based matrix
("matrix") 21, a top surface 22, a bottom surface 24, a front end
26 and a back end 28. The glass-based substrate 20 supports
multiple channel waveguides ("waveguides") 30 defined by respective
waveguide IOX regions 40W (FIG. 1B).
[0040] The example PLC 10 shows by way of example a number of PLC
devices 50 in the form of optical couplers 50C, a switch 50S, and
Y-branches 50Y. The waveguides 30 for these and other types of
devices typically have at least one bend section 60 so that a given
waveguide can be properly routed with respect to another waveguide
or to a PLC component or to another component (e.g., an optical
fiber, a photodetector, an optical coupler, etc.) that can be
placed next to the PLC.
[0041] FIG. 2 is a close-up top-down view of an example bend
section 60 of a waveguide 30 formed in the glass-based substrate
20. The bend section 60 resides between two straight sections 70
and includes opposite ends 62, and inside edge (or concave side)
64, an outside edge (or convex side) 66, a center line CL, a center
of curvature CCB, and a radius of curvature ("bend radius") RB
measured from the center of curvature to the center line. The bend
section 60 has an angular coordinate .theta. and an associated
angular extent .DELTA..theta.. The example bend section 60 of FIG.
2 shows an angular extent .DELTA..theta.90.degree. and has a
"round" or circular curvature, i.e., a constant bend radius RB. The
waveguide 30 can also include other types of bend sections 60,
i.e., different angular directions and extents, a non-constant
radius of curvature, etc., in which case the bend radius RB becomes
a function of the angular coordinate .theta.. The bend section 60
can also be a compensated bend section 60C, as described below. In
one example, the angular extent .DELTA..theta. can be between
85.degree. and 95.degree..
[0042] FIG. 3 is a cross-sectional view in the local (x,y)
coordinates of an example waveguide IOX region 40W for one of the
straight sections 70 of the waveguide 30 as formed in the matrix 21
of the glass-based substrate 20. The corresponding refractive index
profile n.sub.S(x,y) of the waveguide IOX region 40W is shown by
the refractive index contours. The waveguide IOX region 40W and the
immediately surrounding portion of the matrix 21 (which can be
considered the cladding) define the waveguide 30, wherein n.sub.0
can be taken as the cladding index n.sub.c for the waveguide. The
waveguide 30 has a width WW that is slightly larger than the width
WR of the waveguide IOX region 40W, i.e., it includes the cladding
defined by the immediately surrounding portion of matrix 21.
[0043] The example waveguide IOX region 40W of FIG. 3 is
symmetrical about the line x=0 and has a maximum refractive index
n.sub.1=1.528. The matrix 21 has a bulk refractive index
n.sub.0=1.520, so that the refractive index contrast
dn=1.528-1.520=0.006. In other examples, the index contrast dn can
be in the range from about 0.004 to about 0.006. The refractive
index increment .delta.n between contour lines is
.delta.n.apprxeq.4.3.times.10.sup.-4. The refractive index profile
n.sub.S(x,y) of the example waveguide IOX region 40W of FIG. 3
makes the waveguide 30 single mode at an operating wavelength
.lamda., which in an example is a telecommunications wavelength of
800 nm or 1330 or 1550 nm. Other ranges or values may be possible
using the concepts disclosed.
[0044] The waveguide bend loss at a given bend section 60 depends
primarily on the mismatch between the waveguide modes in the
straight sections 70 and the adjacent bend section 60 of the
waveguide 30, and the radiative (leaky) nature of the modes
supported in the bend section. The bend section 60 in the waveguide
30 can be modeled as a straight waveguide that has an equivalent
refractive index profile n.sub.E given by:
n.sub.E(x,y)=n.sub.S(x,y)[1+x/RB].apprxeq.
n.sub.S(x,y)+n.sub.cx/RB
where n.sub.S(x,y) is the refractive index profile of the straight
section 70 of the waveguide 30 under consideration, n.sub.c is the
cladding refractive index and can be taken as the bulk refractive
index n.sub.0. The bend radius is RB, while x is the local lateral
coordinate and y is the local depth coordinate. The equivalent
index profile n.sub.E(x,y) is taken as constant in the local
z-direction over the bend section 60. This equivalent index profile
n.sub.E(x,y) has an inherent bend loss associated with the extra
term n.sub.cx/RB. Note that as the bend radius RB becomes larger,
the bend loss term becomes smaller, and in the limit as RB becomes
infinitely large, the bend section becomes straight and the bend
loss term goes to zero. The term "equivalent refractive index
profile" is used instead of the term "effective refractive index
profile" to avoid confusion with the concept of effective
refractive index used to describe properties of guided modes with
respect to their propagation constants.
[0045] A conventional channel waveguide has the same refractive
index profile n(x,y)=n.sub.S(x,y) in its straight sections as well
as in its bends 60. An aspect of the disclosure is to modify the
refractive index profile n(x,y) in the bend section 60 so that the
bend loss of the waveguide 30 at the bend section is reduced or
eliminated.
[0046] To this end, the waveguide IOX region 40W is modified at the
bend section 60 from n.sub.S(x,y) to
n.sub.C(x,y)=n(x,y)+.DELTA.n(x), where .DELTA.n(x) is a corrective
refractive index term ideally defined by .DELTA.n(x)=Gx, where
G=-n.sub.c/R, where G is a refractive index gradient. The
refractive index profile n.sub.C is referred to hereinafter as the
compensated refractive index profile, and the modified waveguide
IOX region 40W at the bend section 60 becomes a compensated bend
IOX region 40C the defined a compensated bend section 60C, as
introduced and explained in greater detail below.
[0047] The corrective refractive index term .DELTA.n(x) is applied
over the IOX region 40W at the bend section 60 to effect the
transformation to the compensated bend IOX region 40C. The example
IOX region 40W of FIG. 3 resides within a lateral range defined by
x=-20 .mu.m to x=+20 .mu.m. Since the corrective refractive index
term .DELTA.n(x) is subtractive, .DELTA.n(-20 .mu.m)=0 (minimum)
while .DELTA.n(+20 .mu.m)=.DELTA.n.sub.max (i.e., is maximum) while
varying linearly between these two extreme values. Thus, in
examples, the corrective refractive index term .DELTA.n(x) can be
expressed in terms of a shifted coordinate n(x') where
x'=x-.DELTA.x, where in the present example .DELTA.x=-20 .mu.m.
[0048] Table 1 below shows example values of .DELTA.n.sub.max
(x10.sup.-3) for different bend radii RB of 3, 5, 7 9, 11 and 13 mm
and for n.sub.0=n.sub.c=1.5.
TABLE-US-00001 TABLE 1 RB (mm) 3 5 7 9 11 13 .DELTA.n.sub.max
.times. 10.sup.-3 20.0 12.0 8.6 6.7 5.5 4.6
[0049] FIG. 4 is similar to FIG. 3 and shows an example of an ideal
linear modifying IOX region 40M with a linear modifying refractive
index profile n.sub.M(x,y) that can be used to define the
corrective refractive index term .DELTA.n(x').
[0050] FIG. 5 is a plot based on a computer-simulation that
combines a linear modifying IOX region 40M such as shown in FIG. 4
with a waveguide IOX region 40W for a straight portion of a channel
waveguide such as shown in FIG. 3 to arrive at compensated
cross-sectional refractive index profile
n.sub.C(x,y)=n(x,y)+.DELTA.n(x) for the bend section 60 of the
waveguide 30. The compensated refractive index profile n.sub.C(x,y)
is defined at each (x,y) cross-section at each z position along the
(curved) centerline CLW of the bend section 60, as further
explained below in connection with FIGS. 7C, 7D and 7E.
[0051] The corrective refractive index term .DELTA.n(x) is ideally
.DELTA.n(x)=Gx where G=-n.sub.c/RB. In practice, the corrective
refractive index term .DELTA.n(x) will be quasi-linear and only an
approximation of the idealized linear expression. Fortunately, even
a quasi-linear approximation for .DELTA.n(x) is sufficient to
reduce bend loss in bend section 60 as compared to the case where
.DELTA.n(x)=0, i.e., having no corrective refractive index term.
Accordingly, the compensated refractive index profile n.sub.C(x,y)
can be expressed as:
n.sub.C(x,y)=n(x,y)+.DELTA.n.sub.q(x,y)
where .DELTA.n.sub.q(x,y) is a quasi-linear corrective refractive
index term that is at least an approximation to the idealized
corrective refractive index term .DELTA.n(x).
[0052] Forming the Compensated Refractive Index Profile
[0053] It is known in the art that a two-step IOX process involving
first and second different in-diffusing ions (ion I1 and ion I2)
with different diffusivities D.sub.1 and D.sub.2 (where
D.sub.1<D.sub.2) can create in the glass-based substrate 20 two
overlapping IOX regions. Furthermore, the overlapping IOX regions
end up creating a new (third) IOX region with a refractive index
profile n.sub.3(x,y,z) that is substantially the sum of the
refractive index profiles of the two separate IOX regions. Said
differently, two overlapping IOX regions IOX1 and IOX2 can be
thought of as being superimposed, with their respective refractive
index profiles n.sub.1(x,y,z) and n.sub.2(x,y,z) also being
superimposed so that the resulting refractive index profile
n.sub.3(x,y,z).apprxeq.n.sub.1(x,y,z)+n.sub.2(x,y,z), where minor
differences can occur due to non-linear diffusion effects.
[0054] In an example, the glass-based substrate 20 can be an
aluminosilicate glass or a borosilicate glass containing sodium
(e.g., Na.sub.2O) in a sufficient amount for conducting an IOX
process using Na.sup.+ as the out-diffusing native ion from the
matrix 21. In another example, the glass-based substrate 20 can be
an aluminosilicate glass or a borosilicate glass containing lithium
(e.g., Li.sub.2O) in a sufficient amount for conducting an IOX
process using Li.sup.+ as the out-diffusing native ion from the
matrix 21.
[0055] Forming the compensated refractive index profile
n.sub.C(x,y) from the original or base refractive index profile
n.sub.S(x,y) requires modifying the waveguide IOX region 40W at the
given bend section 60. In an example, this can be accomplished by
performing a modifying IOX process having a modifying refractive
index profile n.sub.M(x,y), which introduces the required
refractive index gradient G=-n.sub.c/R (or an approximation
thereto) on top of the original waveguide refractive index profile
n.sub.S(x,y). The modifying IOX process can be performed first,
followed by the waveguide IOX process that forms the waveguide IOX
region 40W having the waveguide refractive index profile
n.sub.S(x,y) to arrive at the compensated refractive index profile
n.sub.C(x,y) for the bend section 60.
[0056] FIG. 6A is an (x,y) cross-sectional view of an example
modifying IOX process that forms a quasi-linear modifying IOX
region 40M in the matrix 21 of the glass-based substrate. The
modifying IOX region 40M has a mid-line ML1 and a modifying
refractive index profile n.sub.M(x,y) in the global Cartesian
coordinates and n.sub.M(x,y) in the local Cartesian coordinates of
the waveguide 30 (yet to be formed in the example shown).
[0057] The first IOX process is shown in the close-up inset IA. The
modifying IOX process is carried using a first IOX medium 90A
having first ions I1 as the replacement or in-diffused ions. In an
example, the first IOX medium 90A comprises a salt bath, such as a
KNO.sub.3 salt bath, wherein the first ions I1 are K.sup.+ ions.
The native or out-diffusing ions are denoted IN, and in an example
can be Na+ or Li+, depending on the composition of the glass-based
substrate 20, as noted above.
[0058] The first IOX process employs a first mask 80A disposed on
the top surface 22 of the glass-based substrate 20. The first mask
80A has a first mask opening 82A. FIG. 6B is a top-down view of the
first mask 80A and an example first opening 82A, which is arcuate
or bend shaped. The first mask opening 82A defines the location and
shape (i.e., distribution or profile) of the modified IOX region
40M. The first mask opening 82A has an inside edge (or concave
side) 84A and an outside edge (or convex side) 86A, and center of
curvature RM measured from a center of curvature CCM to a
centerline CLM. The modifying refractive index profile n.sub.M(x,y)
of the modifying IOX region 40M can be controlled by the main
parameters of the first IOX process, which include temperature,
time, concentration of the first ion I1, as well as by the size and
shape of the first mask opening 82A. A reference line RL that runs
through the center of curvature CCM and through the center of the
first mask opening 82A is shown in FIG. 6B. In an example, the mask
center of curvature CCM is coincident with the bend center of
curvature CCB.
[0059] The modifying refractive index profile n.sub.M(x,y) of the
modifying IOX region 40M becomes increasingly linear with distance
from its origin (x.sub.M, y.sub.M), which can be taken as the (x,y)
location of its maximum refractive index n.sub.1M and which can be
located at or near the top surface 22 of the glass-based substrate
20. The modifying refractive index profile n.sub.M(x, y, z) in the
global (x, y, z) Cartesian coordinates of the glass-based substrate
20 has curved refractive index contours 43M that on the convex side
86A of the waveguide IOX region 40W expand outward and become
farther apart with distance. The local quasi-linear modifying
refractive index profile n.sub.M(x,y) where the compensated bend
section 60C resides (or is to reside) is of main interest.
[0060] FIG. 7 is a plot of the refractive index contrast dn(y)
versus distance y (.mu.m) into the glass-based substrate for an
example blanket K.sup.+-Na.sup.+ IOX process carried out through
the top surface 22 of an aluminosilicate substrate 20 using as the
IOX medium 90A a KNO.sub.3 salt bath (i.e., K concentration of
100%) at a temperature of 380.degree. C. for a diffusion time for a
few hours. The plot shows the results for the transverse electric
(TE) and transverse magnetic (TM) modes. The K.sup.+-Na.sup.+ IOX
process produced a refractive index contrast dn.sub.TE of 0.014 for
TE mode and dn.sub.TM of 0.017 for the TM mode. The profile of the
refractive index contrast dn(y) is relatively linear for y>10
.mu.m
[0061] FIG. 8A is similar to FIG. 6A is an (x,y) cross-sectional
view of an example second IOX process that forms a compensated
waveguide bend section 60C in the quasi-linear modifying IOX region
40M using a second mask 80B having a second mask opening 82B. The
modifying IOX refractive index contours 43M are shown superimposed
with the waveguide IOX refractive index contours 43W. FIG. 8B is a
top-down view of the refractive index contours 43M of the
quasi-linear modifying IOX region 40M and the second mask 80B used
to form the compensated waveguide bend section 60C using the second
IOX process. The location of the first mask opening 82A shown for
reference (dashed outline). The distance between the first and
second mask openings 82A and 82B is D (see FIG. 8A).
[0062] The second IOX process uses a second IOX medium 90B that
contributes a second replacement or in-diffusing ion I2. An example
second ion I2 is silver (Ag.sup.+), which has a much higher
diffusivity then potassium at the same diffusion temperature. The
second IOX process is shown in the close-up inset IB and shows the
second ion I2 exchanging for both the first ion I1 as K.sup.+ and
the native ion IN as Na.sup.+ or Li.sup.+.
[0063] The second mask opening 82B also has an arcuate or bend
shape with a concave side 84B and a convex side 86B, and radius of
curvature RW as measured relative to a centerline CLW. In an
example, the second mask opening 82B is substantially concentric
with the first mask opening 82A, i.e., the two mask openings have
substantially the same center of curvature CCM.
[0064] The modifying IOX region 40M remains substantially
unaffected by the second IOX process remote from where the second
IOX process takes place, i.e., where the modifying and waveguide
IOX regions 40M and 20W do not overlap. This is especially the case
when the second IOX process is carried out at a second diffusion
temperature substantially less than the first diffusion
temperature. In an example, the first diffusion temperature is
380.degree. C. while the second diffusion temperature is
350.degree. C. In any event, it is the quasi-linear portion of the
modifying IOX region 40M that is of main interest.
[0065] The second IOX process is carried out through the second
mask opening 82B in a manner to form the (bended) waveguide IOX
region 40W similar to that of FIG. 3 and which in an example
defines a single mode waveguide 30. However, the (bended) waveguide
IOX region 40W is now superimposed on the quasi-linear modifying
IOX region 40M, for which the modifying refractive index profile
n.sub.M(x,y) serves as the corrective refractive index term
.DELTA.n.sub.q(x,y). In an example, the quasi-linear modifying IOX
region 40M has a mid-line ML2 and a refractive index contrast dn(x)
over the width of the second mask opening 82B. In an example based
on experiments, the refractive index contrast
dn(x)=1.488-1.481=0.006. For second mask opening 82B having a width
in the x-direction of dx, the refractive index gradient G across
the second mask opening at each local (x,y) plane is given by
G=dn(x)/dx. For a width dx=10 .mu.m in an example G=0.006/10
.mu.m=0.0006/.mu.m. The result is the compensated bend section
60C.
[0066] With reference again to Table 1 above, an index contrast
dn=0.006 allows for a bend radius RB for the compensated bend
section 60C as small as about 10 mm. As noted above, the
quasi-linear modifying IOX region 40M representing the corrective
refractive index term .DELTA.n(x) can be adjusted by adjusting at
least one of the first (modifying) IOX process, the second
(waveguide-forming) IOX process, and a distance D between the
mid-lines ML1 and ML2 of the modifying IOX region 40M and the
waveguide IOX region 40W, respectively. In an example, the distance
D can be between 20 .mu.m and 60 .mu.m.
[0067] FIG. 8C is similar to FIG. 2 and is top-down view of a
section of the channel waveguide 30 after forming the compensated
bend section 60C and showing the refractive index profiles at six
local (x,y) cross-sectional planes CX0 through CX5. FIG. 8D shows
the local straight cross-sectional refractive index profile for the
cross-sectional planes CX0 and CX5 taken in the straight portions
of the channel waveguide. FIG. 8E shows the local compensated
cross-sectional refractive index profile n.sub.C(x,y) for the
cross-sectional planes CX1 through CX4 taken in the compensated
bend section 60 of the waveguide 30.
[0068] As can be seen from FIGS. 7C, 7D and 7E, the resulting
compensated refractive index profile n.sub.C(x,y) of the
compensated bend IOX region 40C of the compensated bend section 60C
is defined by the superposition of the quasi-linear modifying IOX
region 40M and the otherwise "straight" waveguide IOX region 40W.
The actual straight sections 70 of the waveguide 30 remain
substantially unaffected, and in one example there is with a
gradual (e.g., adiabatic) transition occurring at the respective
ends 62 of the compensated bend section 60C.
[0069] FIG. 9A is a top-down view of an example PLC 10 formed in
the glass-based substrate 20 and showing an example of two
different waveguides 30 each having compensated bend section 60C,
and also showing the corresponding modifying IOX regions 40M that
run adjacent and substantially parallel to the corresponding
compensated bend sections 60C of the channel waveguides 30 to
define the compensated bend IOX region 40C for each of the
compensated bend sections. Each of the modifying IOX regions 40M
has opposite ends 41M that in one example terminate substantially
in line with the ends 62 of the compensated bend section 60C. In
example, the overall length of the modifying IOX regions 40M can be
made shorter so that the opposite ends 41M do not reach the
location of the ends 62 of the compensated bend section 60C to
provide a more gradual transition between the compensated bend
section and the corresponding straight sections 70 of the given
waveguide 30.
[0070] FIG. 9B is similar to FIG. 9A and shows an example
configuration for the modifying IOX regions 40M that can provide an
adiabatic transition in a transition zone TZ between the
compensated bend sections 60C and the straight portions 70 of the
waveguides 30. The example configuration of the modifying IOX
regions 40M has the opposite ends 41M curving away from its
corresponding waveguide 30.
[0071] FIG. 9C is a top-down view of a portion of a PLC 10 that
includes a PLC device 50 in the form of an optical coupler 50C
having two waveguides 30. FIG. 9C shows an example configuration
for modifying IOX regions 40M to form compensated bend IOX regions
40C for the compensated bend sections 60C in the two waveguides
30.
[0072] Design Considerations
[0073] A consideration in applying the methods of forming the
compensated bend sections 60C as disclosed herein is that the
waveguide bend radius RB of the waveguide 30 is preferably much
larger than the waveguide width WW. For most practical
applications, the waveguide width WW<100 .mu.m while the
waveguide bend radius RB>1 mm, and usually in the range between
5 mm and 30 mm, so that RB>>WW. Further in the example,
WW>5 .mu.m, while in another example, WW>10 .mu.m.
[0074] Another consideration involves ensuring that a given
modifying IOX region 40M substantially only affects a corresponding
bend section 60 of a select waveguide IOX region 40W of a select
waveguide 30 and not other waveguides or the straight sections 70
of the waveguide. The potential issue of the modifying IOX region
40M adversely impacting other waveguides 30 or the straight
sections 70 is readily avoided by computer simulations of the IOX
processes used to form the modifying IOX regions 40M to provide a
good estimate of their respective widths WR and locations so that
they can be properly situated relative to the waveguide 30 to be
compensated, as well as positioned sufficiently remote from any
other modifying IOX regions 40M. Note that in the examples of FIG.
8B and FIGS. 9A, 9B and 9C, the modifying IOX regions 40M were
shaped to follow the shape of the bend section 60 of the waveguide
30. This shaping was discussed above in connection with selectively
forming the first and second mask openings 82A and 82B to have
similar arcuate shapes.
[0075] In practice, the modifying IOX region 40M can have any shape
that contributes to the linear corrective term of the compensated
refractive index profile n.sub.C(x,y) so that the optical loss at
the compensated bend section 60C is less than if the bend section
was unmodified, i.e., had the same refractive index profile
n.sub.S(x,y) as one of the straight sections 70 of the waveguide
30.
[0076] In various examples, the compensated bend sections 60C
provide a reduction in optical loss of at least 5% or at least 10%
or at least 15% or at least 20%. A greater reduction in the optical
loss can be obtained the closer the quasi-linear modifying IOX
region 40M is at providing the exact linear correction term
discussed above.
[0077] Bend Loss Modeling
[0078] The sensitivity of the bend loss compensation to variations
in the corrective refractive index term .DELTA.n(x) of the
corrective refractive index profile
n.sub.C(x,y)=n.sub.S(x,y)+.DELTA.n.sub.q(x,y) was evaluated by
modeling the bend loss of a waveguide 30 having a bend section 60
with a constant bend radius RB and an angular extent
.DELTA..theta., and an initial refractive index n.sub.S(x,y) as
shown in FIG. 3 and with the idealized linear modifying refractive
index profile n.sub.M(x,y)=n.sub.M(x) as shown in FIG. 4.
[0079] FIG. 10A is a plot of optical transmission T versus the
ratio G/G.sub.0, where G.sub.0 is the index gradient=n.sub.0/RB and
G=n.sub.0/(RB.+-..delta.) for a bend radius RW=7.5 mm and .delta.
varying from 0 to (0.5)RB. FIG. 10B is similar to FIG. 10A and
plots the bend loss BL (dB) versus G/G.sub.0. FIGS. 11A and 11B are
the same as FIGS. 10A and 10B respectively, but for a waveguide
bend radius RW3.0 mm. The simulation results show that negligible
loss is introduced due to approximately a +/-20% variation in the
index gradient G at RW=7.5 mm (+/-10% at RB=3 mm), which is a
sufficiently high tolerance for practical applications and for
present manufacturing capabilities.
[0080] Note that the variation in the index gradient G need not be
a variation .delta. in the waveguide radius R but can be a more
general variation in G due to any number of factors, including the
quasi-linear nature of the gradient G associated with variations in
the modifying IOX process and/or the waveguide IOX process.
[0081] For the purposes of describing and defining the present
teachings, it is noted that the terms "substantially" and
"approximately" and "about" are utilized herein to represent the
inherent degree of uncertainty that may be attributed to any
quantitative comparison, value, measurement, or other
representation. The terms "substantially" and "approximately" and
"about" are also utilized herein to represent the degree by which a
quantitative representation may vary from a stated reference
without resulting in a change in the basic function of the subject
matter at issue.
[0082] It is to be understood that variations and modifications can
be made on the aforementioned embodiments without departing from
the concepts of the present disclosure, and further it is to be
understood that such concepts are intended to be covered by the
following claims unless these claims by their language expressly
state otherwise.
* * * * *