US 11,480,736 B2

5

substrate 110 does not include trench 115, such that the
region of top surface 119 shown in FIG. 1 is flat.

FIG. 4 is a plan view of a forked waveguide-coupler 420,
which may replace waveguide 120 in embodiments of
fiber-to-chip coupler 100. Forked waveguide-coupler 420
includes a uniform region 428, an inverse-taper region 424,
and a bifurcation boundary 422 therebetween. Inverse-taper
region 424 has a taper length 425 and includes a first
branch-waveguide 426(1) and a second branch-waveguide
426(2). Uniform region 428 has a uniform width 429, which
is analogous to uniform width 126. At bifurcation boundary
422, uniform region 428 bifurcates into branch-waveguides
426(1) and 426(2). In inverse-taper region 424, branch-
waveguides 426(1) and 426(2) are separated by a branch
separation 427, which adiabatically increases from a mini-
mum distance at bifurcation boundary 422, to a width 423.
In embodiments, width 423 exceeds minimum core diameter
133. In embodiments, forked waveguide-coupler 420
includes a region 402 adjacent to inverse-taper region 424,

in which branch-waveguides 426 maintain a constant width. 2

‘The minimum distance may equal zero.

FIG. 5 is a schematic plan view of a fiber-to-chip coupler
500, which includes substrate 110, forked waveguide-cou-
pler 420, and a capped tapered fiber 570. Capped tapered

fiber 570 is an example of capped tapered fiber 170 and 2

includes optical fiber 130 and a cap 540 thereon. One aspect
of the present embodiments includes the realization that
compared to fiber-to-chip coupler 100, the coupling con-
figuration of fiber-to-chip coupler 500 benefits from
increased mechanical stability, ease of alignment, and tol-
erance to vibrations.

FIG. 6 is a cutaway view of fiber-to-chip coupler 500 in
-sectional plane 6-6', where the viewing direction is
the positive x direction. Cross-sectional plane 6-6' is parallel
to the y-z plane and includes optical axis 131 of optical fiber
130.

FIG. 7 is a cross-sectional view of fiber-to-chip coupler
ctional plane 7-7", which is parallel to the
x-y plane. FIGS. 5-7 are best viewed together in the fol-
lowing description. While branch-waveguides 426 are not in

cross-sectional plane 6, FIG. 6 illustrates a branch-wave-
guide 426(1) with a dashed border for clarity of description.
In the example of FIGS. 5 and 6, forked waveguide-coupler
420 does not include region 402. Along the z axis, tapered
fiber tip 532 and bifurcation boundary 422 are separated by
a distance 537, which is less than taper length 425. In
embodiments, distance 537 equals zero.

Cap 540 is an example of cap 140 and has a cap-length
845. From tapered fiber tip 532, cap 540 extends a distance,

equal to cap-length 545, toward cylindrical section 136. Cap

540 is formed of a material having refractive index n,.
Optical fiber 130 is axially aligned to forked waveguide-

coupler 420. At tapered fiber tip 532, capped tapered fiber

570 has a diameter 573 that is less than width 423. In

embodiments, at least part of tapered-fiber section 134 is

between branch-waveguides 426(1) and 426(2) in a cross-
sectional plane parallel to top surface 119, as shown in FIG.
5. Forked waveguide-coupler 420 has a thickness 421 with
respect to top surface 119. Tapered fiber tip 532 is located at
a height 632 above top surface 119. In embodiments, height
632 exceeds thickness 421. In embodiments, height 632 is
less than thickness 421, such that in cross-sectional plane
6-6', at least part of tapered-fiber section 134 is between
branch-waveguides 426(1) and 426(2). In other embodi-
ments, distance 537 is the distance from bifurcation plane at
which branch separation 427 equals diameter 573 of capped
tapered fiber 570 at tapered fiber tip 532.

6

Forked waveguide-coupler 420 and cap 540 have a top
surface 629 and an outer surface 548 respectively. Branch-
waveguides 426(1,2) have respective inner surfaces 526(1,
2). In embodiments, capped tapered fiber 570 is disposed on
forked waveguide-coupler 420 such that outer surface 548 is
in direct contact with or bonded to at least one of (i) top
surface 629, (ii) inner surface 526(1), (iii) inner surface
526(2), (iv) a first edge surface at an interface between top
surface 629 and inner surface 526(1), and (v) a second edge
surface at an interface between top surface 629 and inner
surface 526(2). For example, outer surface 548 may be
tangential to both a surface of branch-waveguides 426(1,2)
in cross-sectional plane 7-7’, as illustrated in FIG. 7. Outer
surface 548 includes (i) a surface region 548A disposed on
branch-waveguide 426(1), (ii) a surface region 548B dis-
posed on branch-waveguide 426(2), and (iii) a surface
region 548C located between branch-waveguides 426(1)
and 426(2). In cross-sectional plane 7-7', surface region
548C is between surface regions 548A and 548B.

FIG. 7 also illustrates that, in embodiments, capped
tapered fiber 570 intersects a plane that includes top surface
629 at a height equal to thickness 421 above top surface 119.
Furthermore, FIG. 7 illustrates that, in embodiments, forked
waveguide-coupler 420 and capped tapered fiber 570 exhibit
mirror symmetry in a plane 702 that includes optical axis
131 and is perpendicular to top surface 119.

FIG. 5 denotes an end surface 521 of forked waveguide-
coupler 420. Optical fiber 130 extends a distance 523 from
end surface 521 toward bifurcation boundary 422. Herein,
distance 523 is also referred to as an interaction length 523,
as it denotes a length over which coupling between optical
fiber 130 and forked waveguide-coupler 420 occurs. Cross-
sectional plane 7-7' intersects interaction length 523, and
may qualitatively represent more than one cross-section
within interaction length 523 and parallel to the x-y plane.
That is, the transverse dimensions of optical fiber 130, cap
540, and branch separation 427 shown in FIG. 5 depend on
the location of cross-sectional plane 7-7' within interaction
length 523. In embodiments, each of taper lengths 425 and
135 exceeds interaction length 323. In embodiments, at least
one of taper ths 135 and 425 is bet one millimeter
and three millimeters.

In embodiments, interaction length 523 is significantly
greater than a beat length between the coupling waveguides
to ensure efficient optical coupling. In the example of
waveguide coupler 500, the coupling waveguides are end-
section 172 and branch-waveguides 426. This beat length,
herein after z,,, is a function of respective propagation
constants ,7> and B4y5 of optical modes propagating in
end-section 172 and each of branch-waveguides 426:
Zp427!(B,73-Bia26) In embodiments, interaction length 523
is greater than 57,4

In cross-sectional plane 6-6', FIG. 6, outer surface 548 is
oriented at an angle 642 with respect to top surface 629,
and/or a plane that includes top surface 629. In embodi-
ments, angle 642 equals zero such that part outer surface $48
facing top surface top surface 629 is parallel to top surface
629. In fiber-to-chip coupler 500, optical axis 131 of optical
fiber 130 is oriented an angle 638 with respect to top surface
119. In embodiments, angle 638 is greater than or equal to
zero. In embodiments, the shape of outer surface 548 is that
of an outer surface of a truncated cone characterized by an
apex angle. Angle 638 may equal half of the apex angle such
that angle 642 equals zero.

FIG. 8 is a plot 800 showing coupling efficiency 810 of
embodiments of fiber-to-chip coupler 100 as a function of a
range of cap-length values, each of which are example



U.S. Patent Oct. 25, 2022 Sheet 3 of 9 US 11,480,736 B2

500.
TM

ewna| 526(2)
axl

570— Leg 500
Y | | i

Al, x substrate 11
4

FIG. 6

| 4s 626(2)
A 528(1)-2, 1+.
e427»

Lo 702
substrate 110

FIG.7


U.S. Patent Oct. 25, 2022 Sheet 8 of 9 US 11,480,736 B2

—— =.

——

2 wane ee 1.7 pm etch

a
* MF oe eae
¢ So oe
1430 1420
‘ ce
-

FIG. 14


U.S. Patent Oct. 25, 2022 Sheet 5 of 9 US 11,480,736 B2

FIG. 9

FIG. 10


U.S. Patent Oct. 25, 2022 Sheet 4 of 9 US 11,480,736 B2

800

as
o
o

E

fe]
oO

&
oe

oad
Es
ay

Coupling efficiency (%)
Oo
ot

de

she

wr
Oo

50 100 150 200
SU8 cap length (zm)

oO

FIG.8


U.S. Patent Oct. 25, 2022 Sheet 7 of 9 US 11,480,736 B2

58 “ . - . .
SF .
45h 44
fee] we
=, a Va e
#
S35F 4
2 3} :
$
22.5} :
oi
ar d
45h 4
4 ; : ‘ ‘ ‘
20 40 60 80 400 120

Exposure time (s)

FIG. 13


a2) United States Patent
Khan et al.

US011480736!

US 11,480,736 B2
Oct. 25, 2022

(10) Patent No.:
(45) Date of Patent:

(54) FIBER-TO-CHIP COUPLER

ay

(72)

(73)

(*)

1)

(22)
(65)

(60)

61)

(52)

Applicants: The Regents of the University of

Inventors:

Assignees:

Notice:

Appl. No.:

Filed:

US 2021/0215882 Al

Colorado, a body corporate, Denver,
CO (US); Government of the United
States of America as represented by
the Secretary of Commerce,
Gaithersburg, MD (US)

Saeed Khan, Lafayette, CO (US);
Jeffrey Shainline, Boulder, CO (US)

The Regents of the University of
Colorado, a body corporate, De
CO (US); Government of the United
States of America, as represented by
the Sec. of Commerce, Gaithersburg,
MD (US)

Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 0 da

17/144,980
Jan. 8, 2021

Prior Publication Data
Jul. 15, 2021

Related U.S. Application Data
Provisional application No. 62/958,967, filed on Jan,

9, 2020.
Int. Cl.
G02B 6/30 (2006.01)
G02B 6/42 (2006.01)
028 6/293 (2006.01)

.. GO2B 6/305 (2013.01); GO2B 6/29335
(2013.01); G02B 6/4214 (2013.01): GO2B
6/4239 (2013.01)

(58) Field of Classification Search
CPC .. GO2B 6/305; GO2B 6/29335; GO2B 6/4214:
G02B 6/4239

See application file for complete search history.

(56) References Cited
U.S. PATENT DOCUMENTS

4,784,466 A * 11/1988 Khoe G02B 6/4203

385/127

5,044,723 A * 9/1991 MacDonald GOLF 23/2925

385/12

(Continued)

OTHER PUBLICATIONS

Son et al., High-Efficiency Broadband Light Coupling Between
Optical Fibers and Photonic Integrated Circuits, Nanophotonics,
2018; 7(12): pp. 1845-1864.

(Continued)

Primary Examiner — Ellen E Kim
(74) Attorney, Agent, or Firm — Lathrop GPM LLP

(57) ABSTRACT

A fiber-to-chip coupler includes a substrate, a waveguide on
a top surface of the substrate, an optical fiber axially aligned
to the waveguide, and a cap. The waveguide has a uniform
region with uniform width and a tapered-waveguide region
having a width that adiabatically increases from a minimum,
width to the uniform width. The optical fiber has a tapered.
fiber tip having a minimum core diameter, a cylindrical
section having a maximum core diameter, and a tapered-
fiber section therebetween. The optical fiber is located at
least in part above the tapered-waveguide region, and has a
core diameter that adiabatically decreases within a taper
length of the tapered-fiber section. The cap extends from the
tapered fiber tip toward the cylindrical section, is formed of
a second material having a cap refractive index that exceeds
a refractive index of the optical fiber, and includes a cap-
region disposed on the tapered-waveguide region.

19 Claims, 9 Drawing Sheets

l



US 11,480,736 B2

1
FIBER-TO-CHIP COUPLER

CROS

-EFERENCE TO REI
APPLICATIONS

PED

This application claims priority to U.S. provisional patent
application Ser. No. 62/958,967, filed on Jan. 9, 2020, the
disclosure of which is incorporated herein by reference in its
entirety.

GOVERNMENT RIGHTS,

This invention was made with government support under
Grant No. IARPA 16002-D2018-1806040001 awarded by
the Intelligence Advanced Research Projects Activity. The
government has certain rights in the invention.

BACKGROUND,

A major challenge in the field of integrated photonics is
the eflicient coupling of light from a standard optical fiber to
on-chip photonic waveguides and devices. For applications
in digital computing and communications, coupling losses
lead to significant contributions to the link power budget.
For applications in quantum optics and quantum communi-
cation, losses on the order of a few percent may eliminate
any quantum advantage.

SUMMARY OF THE EMBODIMENTS

The problem of fiber-to-chip coupling is difficult because
the fundamental mode of an optical fiber is roughly ten
micrometers in diameter, and the dimensions of the funda-
mental mode of a high-index-contrast waveguide are often
less than one micrometer across. Embodiments herein
include fiber-to-chip couplers configured to efficiently
couple light from the fundamental mode of a single mode
fiber, through a tapering region, into a higher-index cap, and
into a submicron waveguide.

Ina first aspect, a fiber-to-chip coupler includes a sub-
strate, a waveguide on a top surface of the substrate, an
optical fiber axially aligned to the waveguide, and a cap. The
waveguide has (i) a uniform region with uniform width in a
transverse direction parallel to the top surface, and (ii) a
tapered-waveguide region. The tapered-waveguide region
has a width that, in an axial direction perpendicular to the
ses from a mini-
mum width to the uniform width. The minimum width is less
than the uniform width. The optical fiber is axially aligned

to the waveguide, formed of a first material having a fiber 5

refractive index, and has (i) a tapered fiber tip having a
minimum core diameter, (ii) a cylindrical section having a
maximum core diameter, and (iii) a tapered-fiber section
therebetween. The optical fiber is located at least in part

above the tapered-waveguide region, and has a core diam-

eter that, in the axial direction, adiabatically decreases from
the maximum core diameter to the minimum core diameter
within a taper length of the tapered-fiber section. The cap
extends from the tapered fiber tip toward the cylindrical
section, is formed of a second material having a cap refrac-
tive index that exceeds the fiber refractive index, and
includes a cap-region disposed on the tapered-waveguide
region. The tapered-waveguide region is between the cap-
region and the top surface.

In a second aspect, a fiber-to-chip coupler includes a
substrate, a forked waveguide-coupler on a top surface of the
substrate, an optical fiber, and a cap. The forked waveguide-

20

&

2

coupler has (i) a uniform region with uniform width parallel
to the top surface, (ii) a bifurcation boundary at which the
uniform region divides into a first branch-waveguide and a
second branch-waveguide, and (iii) an inverse-taper region
in which a distance between the first and second branch-
waveguides adiabatically increases from a minimum dis-
tance, at the bifurcation boundary, to a positive width. The
optical fiber is formed of a first material having a fiber
refractive index, and has (i) a tapered fiber tip having a
minimum core diameter that is less than the positive width,
(ii) a cylindrical section having a maximum core diameter,
and (iii) an adiabatically-tapered region therebetween. At
least part of the adiabatically-tapered region is located
between the first and second branch-waveguides, and has a
core diameter that decreases from the maximum core diam-
eter to the minimum core diameter within a taper length of
the adiabatically-tapered region. The cap extends from the
tapered fiber tip toward the cylindrical section, and is formed
of a second material having a cap refractive index that
exceeds the fiber refractive index.

In a third aspect, a method for coupling light from an
optical fiber into a waveguide includes adiabatically trans-
ferring light propagating in the optical fiber to a tapered
region of the optical fiber. The method also includes cou-
pling the transferred light, propagating in the tapered region,

to a cladding that is formed on the tapered region and has a

refractive index that exceeds a refractive index of the optical
fiber; and adiabatically coupling the coupled-transferred
light, propagating in the cladding, to a waveguide formed on
a substrate

BRIEF DESCRIPTION OF THE FIGURES,

FIGS. 1-3 are respective schematics of a fiber-to-chip
coupler, in an embodiment.

FIG. 4 is a plan view of a forked waveguide-coupler,
which may replace the waveguide of the fiber-to-chip cou-
pler of FIGS. 1-3, in an embodiment.

FIGS. 5-7 are respective schematics of a fiber-to-chip
coupler that includes the forked waveguide-coupler of FIG.
4, in an embodiment.

FIG. 8isa plot showing coupling efficiency of an embodi-
ment of the fiber-to-chip coupler of FIGS. 5-7 as a function
of cap-length.

FIGS. 9 and 10 are respective schematics of a substrate,
each of which is an example of a substrate of the fiber-to-
chip coupler of FIG. 1.

FIG. 11 illustrates steps for fabricating a capped tapered
fiber, in an embodiment.

FIG. 12 is a microscope image of a capped tapered fiber
fabricated using the steps illustrated in FIG. 11, in an
embodiment.

FIG. 13 isa plot of insertion loss through tapered couplers
and fork couplers as a function of exposure time.

FIG. 14 is an image ofa packaged device that includes an
embodiment of the fiber-to-chip coupler of FIGS. 5-7.

FIG. 15 shows transmission spectrum of a forked wave-
guide-coupler of FIG. 4 before the application of epoxy and
afier curing the epoxy.

FIG. 16 is a flowchart illustrating a method for coupling
light from an optical fiber into a waveguide, in an embodi-
ment.

DETAILED DESCRIPTION OF THE
EMBODIMENTS

FIG. 1 isa schematic of a fiber-to-chip coupler 100, which
includes a substrate 110, a waveguide 120, and a capped


U.S. Patent Oct. 25, 2022 Sheet 9 of 9

US 11,480,736 B2

4 * “" . ~ . ss

before epoxy curing ‘\

AS

a
=

= fh
8 /
2 f
2 f

E

$2.5} /

& ~

i J ‘ater epoxy curing

*
bcd
a

Wavelength (nm)

FIG. 15

1820 «15400-18600 1580 18000-1620

1640
Adiabatically transfer light propagating in an optical fiber fo a tapered region of
the optical fiber.

y

41620
Couple the transferred light, propagating in the tapered region, to a cladding
that is formed on the tapered region and has a refractive index that exceeds a
refractive index of the optical fiber.

Y

1630
Adiabatically couple the coupled-transferred light, propagating in the cladding,
to waveguide formed on a substrate.

FIG. 16

1640

-——1600


US 11,480,736 B2

Sheet 6 of 9

Oct. 25, 2022

U.S. Patent

fm

SU8 dip

HF etch

After developing

UV exposure

FIG. 11

_
|

|

7
a

_
|
| Ce
Co
—
oo
—
co
—

_

.

= |
= ——e

FIG. 12


U.S. Patent Oct. 25, 2022 Sheet 2 of 9 US 11,480,736 B2

126

125 v
Pais Teen : __ Waveguide 120 _
‘4

My . Y A v At
| 124 128

GA 2

FIG. 2

122 1g

substrate 110

FIG. 4


US 11,480,736 B2

7

values of cap-length 145. In the embodiments of fiber-to-
chip coupler 100 characterized in plot 800, taper length 135
is two millimeters and waveguide 120 is formed of silicon
mononitride (SiN). Coupling efficiency 810 has a local
maximum at cap-length value equal to approximately 110
micrometers.

FIG. 9 is a schematic of a substrate 910 with waveguide
120 thereon. Substrate 910 is an example of substrate 110,
FIG. 1. Substrate 910 includes a top surface 919 that forms
a trench 950, which is an example of trench 115. Trench 950
has length 116, depth 117 and a width 954. In embodiments,
width 954 is between ten micrometers and twenty microm-
eters.

FIG. 10 is a schematic of a substrate 1010 with waveguide
120 thereon. Substrate 1010 is an example of substrate 110,
FIG. 1. Substrate 1010 includes a top surface 1019 that
forms a trench 1050, which is an example of trench 115.
Dimensions of trench 1050 include a length 1016, depth 117,
and width 954. Substrate 1010 includes a protrusion 1020

that, in embodiments, supports at least part of tapered- 2

waveguide region 124 of waveguide 120. Protrusion 1020
results in trench 1050 having a concave shape in a horizontal
plane that intersects substrate 1010. Protrusion 1020 has a
top width 1022, a bottom width 1024, and a length 1026.
Length 1016 equals the sum of lengths 116 and 1026.
Bottom width 1024 is less than or equal to width 954 and,
in embodiments, is greater than uniform width 126. In
embodiments, top width 1022 is less than or equal to bottom
width 1024, such that a shape of protrusion 1020 is one of
rectangular, trapezoidal, or triangular in a horizontal plane
that intersects substrate 1010. In embodiments, length 1026
equals or exceeds interaction length 323 and is less than
taper length 125. In embodiments, top width 1022 is at least
three times minimum width 122 of waveguide 120.

‘Tapered Fiber Fabrication

In embodiments, capped tapered fibers 170 are fabricated
with a wet etching procedure using hydrofluoric acid (HF).
FIG. 11 shows fabrication steps 1110, 1120, 1130, and 1140
included in the fabrication of an SU8-capped tapered fiber,
which is an example of capped tapered fiber 170. In step
1110, the acrylic layer an optical fiber is removed, with hot
sulfuric acid for example. The optical fiber is an example of
optical fiber 130, and may be an single-mode optical fiber.
This method of removal is advantageous, as mechanical
stripping leads to microcracks that penetrate and deepen
during subsequent HF etching.

In embodiments, an single-mode optical fiber is etched
using an HF solution, e.g., a 35% HF solution. The etch time
may range between 1.5 hours and 2.5 hours. In embodi-

ments, the etch time is two hours. During this time, both the s

liquid- and vapor-phase HF contribute to the profile of the
etched fiber. The section of the fiber submerged in HF
completely dissolves, while the section above the HF etches
in a tapered profile with a sub-micron tip. The etching of the

section of the fiber above the liquid might result from a s

combination of liquid HF being drawn up the fiber by
capillary action as well as etching by HF vapors. Both the
liquid and vapor etchants decrease in concentration with
height above the liquid, leading to a tapered fiber profile.
In embodiments, a thorough clean with hot sulfuric acid
is performed after the HF etch. The fiber taper was then
inserted into SU8 photosensitive polymer (fabrication step
1120 in FIG. 11) and subsequently exposed using 365 nm
ultraviolet (UV) light. To produce a thin cladding on the
surface of the fiber only around the tip (i.e., the cap), the SU8
polymer is exposed by sending UV light inside the fiber
(fabrication step 1130 in FIG. 11) using a fiber-coupled LED

3

&

&

8

the emits an ultraviolet light (e.g., center wavelength 4.=365
nm) and output power of between eight and twelve milli-
watts (e.g. ten milliwatts). In embodiments, only a fraction
of the LED power is available for the exposure due to the
core diameter mismatch between the LED’s multimode
patch cord and the single-mode optical fiber. The core
diameter of the multimode patch cord was 400 «um, and
single-mode optical fiber was coupled to this multimode
patch cord using a bare fiber adaptor. As the evanescent tail
of the UV guided mode leaks outside the SiO, of the fiber
taper, more SUS polymer is gradually exposed down the
length of the taper, leading to a polymer cap (e.g., cap 140)
of gradually increasing thickness. In fabrication step 1140,
the fiber is developed, leaving an SUS cap on the tip without
an abrupt fiber/SU8 interface. FIG. 12 shows a microscope
image of a completed capped tapered fiber, which is an
example of caped tapered fiber 170.

Experimental Characterization

Insertion loss between cap 140 and waveguide 120
depends on the thickness profile of cap 140. This profile can
be adjusted by the UV exposure time, with longer exposure
leading to a thicker, longer cap. FIG. 13 is a plot of insertion
loss through taper couplers (examples of waveguide 120)
and fork couplers (examples of forked waveguide-coupler
420) as a function of exposure time. The error bars in FIG.
13 were obtained by manufacturing multiple tapers in dif-
ferent fabrication batches and measuring their performance
when coupling to the same waveguide structure. The error
bars represent the standard deviation of these measurements.
Variation in coupler loss was due to wet etching and manual
application of the SUS on the fiber by dipping. The amount
of SU8 remaining on the fiber taper before exposure and
development depends on the speed at which the fiber is
removed from the SU8. Fast removal results in more SU8
remaining on the fiber. This excess SUS gathers into a bead
at the fiber tip. This bead is difficult to remove with the
standard developing procedure and leads to decreased cou-
pling efficiency. One important insight from FIG. 13 is that
a relatively broad range of exposure times provides similar
coupling, indicating robustness with respect to this process-
ing condition.

Packaging

For many applications, coupling light from a fiber to a
waveguide is only useful if the assembly can be packaged in
a manner that enables deployable systems. This is often
accomplished by aligning fibers to coupling structures and
fixing them in piace with epoxy. However, epoxies useful for
such packaging have an index of refraction very close to that
of the SU8 used as cap 140 and would lead to high losses.
Accordingly, in embodiments, capped tapered fiber 170 is
fixed with epoxy in a groove etched in a separate carrier
chip, which may be formed of silicon. The tapered region of
the fiber extends beyond the carrier chip so that it can be
placed on the waveguide device chip.

In embodiments, the carrier chips are fabricated by etch-
ing channels using deep reactive ion etching. The transverse
dimensions—depth and width—of each channel n be
between 30 jm and 70 um. The tapered fibers are epoxied to
the carrier chips. After placing the fiber into the channel,
epoxy is delivered to the channel through a delivery com-
partment etched on the same fiber carrier chip. For packag-
ing, the device chip is first mounted, with an epoxy for
example, on a carrier substrate such as a glass slide. When
aligning a capped tapered fiber 170 to the device chip (e.g.,
substrate 110 with waveguide 120 thereon), the carrier chip
is held by vacuum suction. After capped tapered fiber 170 is
aligned to the device chip, the carrier chip is fixed in place



US 11,480,736 B2

11

13. The fiber-to-chip coupler of claim 1, the substrate
being formed of an oxide material.

14. The fiber-to-chip coupler of claim 1, a refractive index
of the waveguide exceeding a refractive index of the sub-
strate.

15. The fiber-to-chip coupler of claim 1, the waveguide
being formed of silicon nitride.

16. A fiber-to-chip coupler comprising:

a substrate having a top surface;

a forked waveguide-coupler on the top surface, having (i)

a uniform region with uniform width parallel to the top
surface, (ii) a bifurcation boundary at which the uni-
form region divides into a first branch-waveguide and
a second branch-waveguide, and (iii) an inverse-taper
region in which a distance between the first and second
branch-waveguides adiabatically increases from a
minimum distance, at the bifurcation boundary, to a
positive width;

an optical fiber, formed of a first material having a fiber

refractive index, and having (i) a tapered fiber tip 2

having a minimum core diameter that is less than the
positive width, (ii) a cylindrical section having a maxi-
mum core diameter, and (iii) an adiabatically-tapered
region therebetween, at least part of which is located

0

12

between the first and second branch-waveguides, and
having a core diameter that decreases from the maxi-
mum core diameter to the minimum core diameter
within a taper length of the adiabatically-tapered
region; and

a cap extending from the tapered fiber tip toward the

cylindrical section, and being formed of a second
material having a cap refractive index that exceeds the
fiber refractive index.

17. The fiber-to-chip coupler of claim 16, the cap having
acap length along an optical axis of the optical fiber, the cap
length exceeding a beat length between a fundamental mode
of the adiabatically-tapered region and a fundamental mode
of the inverse-taper region.

18. The fiber-to-chip coupler of claim 16, the cap includ-
ing (i) a first surface region disposed on the first branch-
waveguide, (ii) a second surface region disposed on second
first branch-waveguide, and (iii) a third surface region
located between the first branch-waveguide and the second
branch-waveguide.

19. The fiber-to-chip coupler of claim 16, the cap refrac-
tive index exceeding a refractive index of the substrate.

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US 11,480,736 B2

Sheet 1 of 9

Oct. 25, 2022

U.S. Patent

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US 11,480,736 B2

Page 2
(56) References Cited
U.S. PATENT DOCUMENTS.

5,461,692 A * 10/1995 Nagel o G02B 6/03627
385/38
7,099,535 B2* 8/2006 Bhagavatula .. GO2B 6/4206
385/124
9,122,037 B2* 9/2015 Shastri ....... ... B32B 37/16
10,488,596 B2* 11/2019 Akiyama ... .... GO2B 6/12
10,514,506 B2* 12/2019 Brusberg ... .. GO2B 6/3636
2016/0103279 AL* 4/2016 Budd . GO02B 6/12004
438/31
2016/0327748 AL* 11/2016 Stern G02B 6/305,
2017/0059780 Al* 3/2017 Budd G02B 6/245

OTHER PUBLICATIONS
Jagerska, Dispersion Properties of Photonic Crystals and Silicon
Nanostructures Investigated by Fourier-Space Imaging, https://www.
researchgate.net/publication/49460024. 153 pages.

* cited by examiner


US 11,480,736 B2

3
tapered fiber 170, which includes an optical fiber 130, and a
cap 140. FIG. 2 is a plan view of waveguide 120. FIG. 3 is
a cross-sectional view of fiber-to-chip coupler 100. FIGS.
1-3 are best viewed together in the following description.

The cross-section illustrated in FIG. 2 is parallel to a
plane, hereinafter the x-z plane, formed by orthogonal axes
AJ and A3, which are each orthogonal to an axis A2. The
cross-section illustrated in FIG. 3 is parallel to a plane,
hereinafter the y-z plane, formed by orthogonal axes A2 and
A3, which are each orthogonal to axis Al. Herein, the x-y
plane is formed by orthogonal axes Al and A2, and planes
parallel to the x-y plane are referred to as transverse planes.
Unless otherwise specified, heights of objects herein refer to
the object’s extent along axis A2. Herein, a reference to an
axis x, y, or z refers to axes Al, A2, and A3 respectively.
Also herein, a horizontal plane is parallel to the x-z plane, a
width refers to an object’s extent along the x axis, and a
vertical direction is along the y axis.

Waveguide 120 is on a top surface 119 of substrate 110.
Waveguide 120 includes a uniform region 128 having (i) a
uniform width 126 parallel to top surface 119, and (ii) a
height 127. In embodiments, height 127 is between 200 nm
and 240 nm. Waveguide 120 also includes a tapered-wave-
guide region 124 having a width that adiabatically increases
from minimum width 122 to uniform width 126. Minimum
width 122 is less than uniform width 126. Tapered-wave-
guide region 124 has a taper length 125.

Optical fiber 130 is axially aligned to waveguide 120 and
is formed of a first material having a refractive index n,. In
embodiments, the first material is silicon dioxide. Optical
tip 132 having a minimum core
diameter 133, (ii a cylindrical section 136 having a maxi-
mum core diameter 137, and a tapered-fiber section 134
therebetween. Tapered-fiber section 134 is located above
tapered-waveguide region 124, and has a core diameter that
adiabatically decreases from maximum core diameter 137 to
minimum core diameter 133 within a taper length 135 of
tapered-fiber section 134, In embodiments, refractive index
n, exceeds a refractive index nj, of substrate 110, which
enables total internal reflection of light propagating in
waveguide 120 from top surface 119.

Optical fiber 130 has an optical axis 131, which is
oriented at an angle 138 with respect to top surface 119. In
embodiments, angle 138 is greater than equal to zero. For
‘example, angle 138 may be between three degrees and thirty
degrees. In embodiments, maximum core diameter 137 and
taper length 135 determine angle 138. Denoting maximum
core diameter 137 and taper length 135 as D and L respec-
tively, angle 138 may be equal to arcsin(D/2L). In embodi-

ments, optical fiber 130 is a single-mode optical fiber at s

free-space wavelengths between 1530 nanometers and 1610
nanometers.

A cap end-region 142 of cap 140 is disposed directly on
tapered-waveguide region 124. Tapered-waveguide region

124 is between cap end-region 142 and top surface 119. Ins

embodiments, cap-length 145 is between twenty-five
micrometers and two hundred micrometers. FIG. 1 denotes
an end-section 172 of capped tapered waveguide 170 that
includes cap-end region 142.

From tapered fiber tip 132. cap 140 extends a cap-length
145 toward cylindrical section 136, and is formed of a
second material having a refractive index n, that exceeds the
refractive index n,. One aspect of the present embodiments
includes the realization that n,’s exceeding n, enables efli-
cient optical coupling between capped tapered fiber 170 and
waveguide 120 by preventing leakage of optical power into
a continuum of substrate modes of substrate 110. When n,

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exceeds n,, the effective index of an optical mode (or
supermode) of cap end-region 142 exceeds the refractive
index of substrate 110 in spatial regions where the evanes-
cent field of the optical mode reaches top surface 119, which
prevents the aforementioned optical power leakage.

Waveguide 120 has an end surface 121, illustrated in FIG.
3. Above waveguide 120, a thickness of cap 140 increases
from a thickness 343 above end surface 121 to a thickness
344 at tapered fiber tip 132. In embodiments, thickness 343
is less than ten nanometers and thickness 334 is between 0.1
micrometers and 1.0 micrometers. Cap 140 has an outer
surface 148, which in embodiments includes a region 348
that is at least one of (i) parallel to and (ii) disposed on a top
surface 129 of waveguide 120, as illustrated in FIG. 3. In
embodiments, region 348 is disposed directly on top surface
129. In embodiments, the shape of outer surface 148 is an
outer surface of a truncated cone characterized by an apex
angle. Angle 138 may equal half of the apex angle such that
region 348 is parallel to top surface 129.

‘Above top surface 129 and along the z axis, optical fiber
130 and cap 140 extend respective distances 323 and 324
from end surface 121. Distance 324 may exceed distance
323. In embodiments, the length of region 348 in the z

5 direction equals distance 324. Herein, distance 323 is

referred to as an interaction length 323, as it corresponds to
the length of waveguide 120 over which coupling between
capped tapered fiber 170 and waveguide 120 occurs. In
embodiments, each of taper lengths 125 and 135 exceeds
interaction length 323. In embodiments, at least one of taper
lengths 125 and 135 is between one millimeter and three
millimeters.

In embodiments, interaction length 323 is significantly
greater than a beat length between the coupling waveguides
to ensure efficient optical coupling. In the example of
fiber-to-chip coupler 100, the coupling waveguides are end-
section 172 and tapered-waveguide region 124. This beat
length, herein afier z,,, is a function of respective propaga-
tion constants f},73 and f.,2, of optical modes propagating in
end section 172 and tapered-waveguide region 124: z,, ,=27/
(Bi72-Bi24)- In embodiments, interaction length 323 is
greater than 5,,.

Cap 140 covers at least part of tapered-fiber section 134.
In embodiments, cap-length 145 is greater than or equal to
taper length 135, such that cap 140 covers all of tapered-
fiber section 134, and may also extend to cover part of
cylindrical section 136. While such caps are easier to
fabricate than those that only partially cover tapered-fiber
section 134, they may exhibit more loss.

In embodiments, each of substrate 110, waveguide 120,
and cap 140 is formed of a dielectric, a semiconductor, or a
combination thereof. Substrate 110 may be formed of an
oxide, waveguide 120 may be silicon or silicon nitride, and
cap 140 may be formed of an epoxy-based photoresist, such
as SU-8, silicon nitride, or amorphous silicon. In embodi-
ments, waveguide 120 has a rectangular cross-section.

In embodiments, top surface 119 forms a trench 115,
illustrated in FIGS, 1 and 3. A technical benefit of trench 115
is to further reduce leakage to substrate modes by allowing
the evanescent tail of a mode (or supermode) of cap end-
region 142 to extend beyond the material structure without
coupling to undesired modes of substrate 110. Trench 115
has a length 116 and a depth 117 in z and y directions
respectively. In embodiments, depth 117 is between 1.5
micrometers and 2.0 micrometers. In the x direction, trench
115 has a width that, in embodiments, is greater than or
equal to maximum core diameter 137. In embodiments,


US 11,480,736 B2

9

on the carrier substrate. To perform this step, an epoxy is
applied on the carrier substrate close to the carrier chip. Due
to surface tension, the epoxy is drawn between the carrier
chip and the carrier substrate and spreads uniformly under
the carrier chip. Height adjustment to realign capped tapered
fiber 170 is often needed after the application of the epoxy.
The alignment and packaging procedure are conducive to
automation with machine vision and feedback from optical
‘transmission.

FIG. 14(a) shows a packaged device, while FIG. 14(b)
shows a zoom of the s-shaped 1.8 mm-long SiN waveguide
with forked waveguide-couplers and tapered fibers on either
side. FIG. 14(c) shows a tapered fiber 1430 aligned on the
forked waveguide-coupler 1420 in the packaged device.
Tapered fiber 1430 is capped with a cap 1440 formed of
SU8. Forked waveguide-coupler 1420, tapered fiber 1430,
and cap 1440 are examples of forked waveguide-coupler
420, optical fiber 130, and cap 140 respectively.

These devices were measured before and after epoxy was
applied and cured. FIG. 15 shows the transmission spectrum
of forked waveguide-coupler 1420 before the application of
epoxy undemeath the carrier chip and after the epoxy has
been cured on both sides. The spectrum shifts due to a slight
adjustment in the longitudinal position of tapered fiber 1430
relative to forked waveguide-coupler 1420. This adjustment
can be pre-compensated prior to delivering and curing the

art illustrating a method 1600 for
coupling light from an optical fiber into a waveguide.
Method 1600 includes steps 1610, 1620, and 1630. In
embodiments, method 1600 is implemented with fiber-to-
chip coupler 100, FIGS. 1-3, or fiber-to-chip coupler 500,
FIGS. 4-7.

Step 1610 includes adiabatically transferring light propa-
gating in the optical fiber to a tapered region of the optical
fiber. In an example of step 1610, light propagating in
cylindrical section 136 of optical fiber 130 adiabatically
transfers to tapered-fiber section 134 of optical fiber 130.

Step 1620 includes coupling the transferred light, propa-
gating in the tapered region, to a cladding that is formed on
the tapered region and has a refractive index that exceeds a
refractive index of the optical fiber. Ina first example of step
1620, light propagating in tapered-fiber section 134 of
optical fiber 130 is adiabatically coupled to cap 140 of
capped tapered fiber 170, FIG. 1. In a second example of
step 1620, light propagating in tapered-fiber section 134 of
optical fiber 130 is adiabatically coupled to cap 540 of
capped tapered fiber 570, FIG. 5.

Step 1630 includes adiabatically coupling the coupled-

transferred light, propagating in the cladding, to a wave- 5

guide formed on a substrate. In a first example of step 1630,
fiber-to-chip coupler 100 is used to implement method 1600.
In this first example, light propagating in cap 140 of capped
tapered fiber 170 is adiabatically coupled to waveguide 120.

Ina second example of step 1630, fiber-to-chip coupler 500. 5

is used to implement method 1600. In this second example,
light propagating in cap 540 of capped tapered fiber 570 is
adiabatically coupled to forked waveguide-coupler 420.
‘Changes may be made in the above methods and systems
without departing from the scope of the present embodi-
ments. It should thus be noted that the matter contained in
the above description or shown in the accompanying draw-
ings should be interpreted as illustrative and not in a limiting
sense. Herein, and unless otherwise indicated the phrase “in
embodiments” is equivalent to the phrase “in certain
embodiments,” and does not refer to all embodiments. The
following claims are intended to cover all generic and

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specific features described herein, as well as all statements
of the scope of the present method and system, which, asa
matter of language, might be said to fall therebetween,

What is claimed is:

1. A fiber-to-chip coupler comprising:

a substrate having a top surface;

a waveguide on the top surface, having (i) a uniform
region with uniform width in a transverse direction
parallel to the top surface, and (ii) a tapered-waveguide
region having a width that, in an axial direction per-
pendicular to the transverse direction, adiabatically
increases from a minimum width to the uniform width;

an optical fiber axially aligned to the waveguide, formed
of a first material having a fiber refractive index, and
having (i) a tapered fiber tip having a minimum core
diameter, (ii) a cylindrical section having a maximum
core diameter, and (iii) a tapered-fiber section therebe-
tween, located at least in part above the tapered-
waveguide region, and having a core diameter that, in
the axial direction, adiabatically decreases from the
maximum core diameter to the minimum core diameter
within a taper length of the tapered-fiber section; and

a cap extending from the tapered fiber tip toward the
cylindrical section, being formed of a second material
having a cap refractive index that exceeds the fiber
refractive index, and including a cap-region disposed
on the tapered-waveguide region, the tapered-wave-
guide region being between the cap-region and the top
surface.

2. The fiber-to-chip coupler of claim 1, the tapered-fiber
section including a tapered-fiber subsection not covered by
the cap-region, the top surface forming a trench beneath at
least part of the tapered-fiber subsection.

3. The fiber-to-chip coupler of claim 2, a depth of the
trench with respect to the top surface being between 1.5
micrometers and 2.0 micrometers.

4. The fiber-to-chip coupler of claim 2, the trench includ-
ing a region adjacent to the waveguide in the transverse
direction.

5. The fiber-to-chip coupler of claim 1, the cap refractive
index exceeding a refractive index of the substrate.

6. The fiber-to-chip coupler of claim 1, an outer surface of
the cap including a surface-region in direct contact with a
top surface of the waveguide.

7. The fiber-to-chip coupler of claim 6, in a cross-
sectional plane perpendicular to the top surface and inter-
secting both the waveguide and an optical axis of the optical
fiber, the surface-region being parallel to the top surface and
having a length equal to a length of the cap directly above
the waveguide.

8. The fiber-to-chip coupler of claim 1, the cap having a
cap length along an optical axis of the optical fiber, the cap
length exceeding a beat length between a fundamental mode
of the optical fiber and a fundamental mode of the wave-
guide.

9. The fiber-to-chip coupler of claim 8 the beat length
being equal to 27/(6B,,), where Band B,, are respective
propagation constants of the fundamental mode of the
adiabatically-tapered region and the fundamental mode of
the tapered-waveguide region.

10. The fiber-to-chip coupler of claim 1, the cap being
formed of a polymer material.

11. The fiber-to-chip coupler of claim 10, the polymer
material being an epoxy-based negative photoresist.

12. The fiber-to-chip coupler of claim 11, the epoxy-based
negative photoresist being SU-8.