U.S. Patent Aug. 10, 2021 Sheet 18 of 20 US 11,086,193 B1

2um

laser

FIG. 18


US 11,086,193 BI

29

4. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, wherein the substrate com-
prises silicon.

5. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, wherein the dielectric layer
comprises silicon dioxide.

6. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, wherein the dielectric layer
comprises a gas.

7. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, wherein the input light is
pulsed and comprises a central wavelength from 1200 nm to
2500 nm.

8. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, wherein the supercontinuum
light comprises a wavelength from 600 nm to 3000 nm.

9. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, wherein the second har-
monic light comprises a central wavelength from 600 nm to
1250 nm.

10. A detector for detecting a frequency of a carrier
envelope offset, the detector comprising:

a self-referencing nonlinear frequency converting pho-

tonic waveguide comprising:

a substrate;

a dielectric layer disposed on the substrate with an
input terminus at which input light propagates
toward an output terminus opposingly disposed from
the input terminus:

rs

input terminus on the dielectric layer and compris-

ing:

Al,Ga,As,, wherein x is a number from 0 atomic
percent (at. %) to 99.9 at %; y is a number from
1 at. % to 55 at. %, and z is a number from 1 at.
% to 55 at. %, based on a total of all atoms in the
supercontinuum generator section;

a supercontinuum input optical taper disposed at the
input terminus and that receives input light. the
supercontinuum input optical taper tapering
toward the input terminus;

a supercontinuum nonlinear optical member in opti-

cal communication with the supercontinuum input

optical taper and that receives the input light from
the supercontinuum input optical taper and pro-
duces supercontinuum light from the input light,
the supercontinuum light spanning an optical
octave with respect to the input light; and
supercontinuum output optical taper in optical
communication with the supercontinuum nonlin-
ear optical member and that receives the super-
continuum light and the input light from the
supercontinuum nonlinear optical member and
propagates the supercontinuum light and the input
light from the supercontinuum generator section,
the supercontinuum output optical taper tapering
away from the input terminus,

a first height H1 relative to a surface of the dielectric
layer on which the supercontinuum generator sec-
tion is disposed;

a first width W1 of supercontinuum nonlinear optical
member orthogonal to a direction propagation of
supercontinuum light in supercontinuum genera-
tor section; and

a second harmonic generator section disposed on the
supercontinuum output optical taper and the dielec-
tric layer and that extends on the dielectric layer

.

supercontinuum generator section disposed at the 3

0

20

2

60

30

from the supercontinum output optical taper

towards the output terminus and comprising:

Al,Ga,As, that provides birefringent modal phase
matching, wherein x is a number from 0 atomic
percent (at. %) to 99.9 at. %; y is a number from
1 at. % to 55 at. %, and z is a number from 1 at
% to 55 at. %, based on a total of all atoms in the
second harmonic generator section;

a second harmonic input optical taper disposed on
the supercontinuum output optical taper and that
receives the supercontinuum light and the input
light from the supercontinuum output optical
taper, the second harmonic input optical taper
tapering toward the supercontinuum output optical
taper,

a second harmonic nonlinear optical member in
optical communication with the second harmonic
input optical taper and that receives the supercon-
tinuum light and the input light from the second
harmonic input optical taper and produces second
harmonic light from the supercontinuum light and
the input light, the second harmonic light com-
prising a second harmonic of the supercontinuum
light:

a second harmonic output optical taper disposed at
the output terminus in optical communication with
the second harmonic nonlinear optical member
and that receives the second harmonic light, the
supercontinuum light, and the input light from the
second harmonic nonlinear optical member and
co-propagates the second harmonic light, the
supercontinuum light, and the input light from the
second harmonic generator section toward the
output terminus, the second harmonic output opti-
cal taper tapering away from the second harmonic
nonlinear optical member toward the output ter-
minus;

a second height H2 relative to the surface of the
dielectric layer on which the second harmonic
generator section is disposed;

a second width W2 of supercontinuum nonlinear
optical member orthogonal to a direction propa-
gation of second harmonic light in the second
harmonic generator section,

such that the first height H1 is greater than or equal to
the second height H2, and the first width W1 is less
than or equal to second width W2;

a wavelength splitter in optical communication with the
self-referencing nonlinear frequency converting pho-
tonic waveguide and that:
receives the output light from the self-referencing non-

linear frequency converting photonic waveguide;

and
produces combination signal and broadened spectrum
from the output light:

a photodetector in optical communication with the wave-
length splitter and that receives the combination signal
from the wavelength splitter and produces photosignal
from the combination signal;

a spectrum analyzer in electrical communication with the
photodetector and that receives the photosignal and
produces an analyzer signal from the photosignal; and

an electrical stabilizer in electrical communication with
the spectrum analyzer and that receives the analyzer
signal from the spectrum analyzer, produces a feedback
signal from the analyzer signal, and communicates the


U.S. Patent Aug. 10, 2021 Sheet 12 of 20 US 11,086,193 B1

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FIG. 12


US 11,086,193 BI

3

and that receives the second harmonic light, the supercon-
tinuum light, and the input light from the second harmonic
nonlinear optical member and co-propagates the second
harmonic light, the supercontinuum light, and the input light
from the second harmonic generator section toward the
output terminus, the second harmonic output optical taper
tapering away from the second harmonic nonlinear optical
member toward the output terminus; a second height H2
relative to the surface of the dielectric layer on which the
second harmonic generator section is disposed; a second
width W2 of supercontinuum nonlinear optical member
orthogonal to a direction propagation of second harmonic
light in the second harmonic generator section, such that the
first height H1 is greater than or equal to the second height
H2, and the first width W1 is less than or equal to second
width W2, a wavelength splitter in optical communication
with the self-referencing nonlinear frequency converting
photonic waveguide and that: receives the output light from
the self-referencing nonlinear frequency converting pho-
tonic waveguide; and produces combination signal and
broadened spectrum from the output light; a photodetector in
optical communication with the wavelength splitter and that
receives the combination signal from the wavelength splitter
and produces photosignal from the combination signal; a
spectrum analyzer in electrical communication with the
photodetector and that receives the photosignal and pro-
duces a analyzer signal from the photosignal; and an elec-
trical stabilizer in electrical communication with the spec-
trum analyzer and that receives the analyzer signal from the

spectrum analyzer, produces a feedback signal from the 3

analyzer signal, and communicates the feedback signal to a
laser source that produces the input light based on the
feedback signal.

Disclosed is a process for self-referencing nonlinear fre-

quency conversion with the self-referencing nonlinear fre- 3

quency converting photonic waveguide of claim 1, the
process comprising: receiving the input light by the super-
continuum input optical taper, communicating the input light
from the supercontinuum input optical taper to the super-
continuum nonlinear optical member, receiving the input
light by the supercontinuum nonlinear optical member sub-
jecting, by the input light, the input light to anomalous group
velocity dispersion; producing, by the supercontinuum non-
linear optical member, the supercontinuum light from the
input light in response to subjecting the input light to the
anomalous group velocity dispersion and birefringent phase
matching in the supercontinuum nonlinear optical member;
communicating the supercontinuum light and the input light
from the supercontinuum nonlinear optical member to the
supercontinuum output optical taper; receiving the input
light and the supercontinuum light by the supercontinuum
output optical taper and communicating the supercontinuum
light and the input light from the supercontinuum output
optical taper to the second harmonic input optical taper;
receiving the input light and the supercontinuum light by the
second harmonic input optical taper and communicating the
supercontinuum light and the input light from the second
harmonic input optical taper to the second harmonic non-
linear optical member; receiving the supercontinuum light
and the input light by the second harmonic nonlinear optical
member and producing, by the second harmonic nonlinear
optical member, the second harmonic light from the super-
continuum light and the input light; and communicating the
second harmonic light, the supercontinuum light, and the
input light from the self-referencing nonlinear frequency
converting photonic waveguide as output light; producing a
feedback signal based on the output light; and producing the

2

40

&

4

input light that is feedback stabilized by the feedback signal
prior to receiving the input light by the supercontinuum
input optical taper to perform self-referencing nonlinear
frequency conversion.

BRIEF DESCRIPTION OF THE DRAWINGS.

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike.

FIG. 1 shows a plan view of a self-referencing nonlinear
frequency converting photonic waveguide:

FIG. 2 shows a perspective view of a self-referencing
nonlinear frequency converting photonic waveguide;

FIG. 3 shows, in panel A, a cross-section along line A-A
of the self-referencing nonlinear frequency converting pho-
tonic waveguide shown in FIG. 1; in panel B, a cross-section
along line B-B of the self-referencing nonlinear frequency
converting photonic waveguide shown in FIG. 1; and in
panel C a cross-section along line C-C of the self-referenc-
ing nonlinear frequency converting photonic waveguide
shown in FIG. 1;

FIG. 4 shows, in panel A, a cross-section along line D-D
of the self-referencing nonlinear frequency converting pho-
tonic waveguide shown in FIG. 1; and in panel B a cross-
section along line E-E of the self-referencing nonlinear
frequency converting photonic waveguide shown in FIG. 1

FIG. 5 shows a self-referencing nonlinear frequency con-
verting photonic waveguide;

FIG. 6 shows a self-referencing nonlinear frequency con-
verting photonic waveguide,

FIG. 7 shows a detector;

FIG. 8 shows a flow chart for self-referencing nonlinear
frequency conversion;

FIG. 9 shows a suspended Al,Ga,As, on silicon platform.
(a), (b) SEM images of a waveguide facet produced by dry
etching; (c) a processed wafer prior to die release, wherein
an Al,Ga,As. film was bonded over the entire surface, and
devices were fabricated in a middle dashed portion; (d)
fabrication flow for six processing steps; (e) focus-stacked
image of a microring resonator die after release from the
wafer;

FIG. 10 shows microring resonators. (a) SEM image of a
resonator, showing pulley-coupler region and sidewall
roughness of a ring; (b) dark-field optical micrograph of the
top view of a fabricated resonator. Suspended regions
include waveguide features on the substrate. (c) Ring wave-
guide width versus propagation loss for resonances taken at
4=1564, 1556, and 1592 nm, from left to right. (4) Compiled
propagation loss data versus wavelength. The first two data
points (1260 and 1592 nm) use the intrinsic rather than
loaded Q. For the others, the signal was AC-coupled. (e)-(i)
Experimental and fitted traces for the resonance considered
in each datapoint of subplot (d). W, width of ring waveguide;
R, radius of ring resonator; Q, loaded quality factor.

FIG. 11 shows characterization of passive suspended
Al,Ga,As, features. (a) Loss-per-edge-coupler at different
wavelengths. Top inset, inverted taper geometry, showing tip
width wt and taper sections; bottom inset, optical micro-
graph of an inverted taper edge coupler; (b) bend radius
versus single-bend loss at 4=4.6 um for a 1.4 jum-wide
waveguide; (c) measured MMI power splitter efficiency for
several variations on the length and width of the multimode
propagation section; (d), (e) optical micrographs of cutback
structures used to measure single-bend and single-splitter
loss, respectively. Top inset of (e): zoom view of consecutive


U.S. Patent Aug. 10, 2021 Sheet 3 of 20 US 11,086,193 B1

wm wi
1-202
+ 205
HI
+20] X cl
+ y
( 10 W2
B) 200
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pul yO
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A wn w2— x a
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205
HI
avs T28
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pat K J

FIG. 3


US 11,086,193 BI

25

uniformly, causing phase-errors. The SiO, facets are etched
with ICP-RIE using CHF, and O, gases, and the chips are
released front the wafer using a deep-RIE etch of the Si
substrate. A scanning electron micrograph (SEM) of the
waveguide at the chip facet is shown in FIG. 17(c). We
estimate the sidewall roughness as 4.5 nm RMS using a
top-view SEM. Finally, the chips are cleaned with HCI (1.1
mol/L) for 10 s to minimize the As—As bonds.

To measure the SHG conversion efficiency, we use a CW
and tunable 2 laser to pump the waveguides in the TE-
polarized mode, as shown in FIG, 18. A ~1% splitter is used
to monitor the input power, and tensed fibers couple light on
and off the chip. After the output fiber, a wavelength-
division multiplexer (WDM) splits the TM-polarized signal
and TE-polarized pump light so they can be monitored, on
separate photodetectors. Similar setups are used to measure
transmission at 980 nm, 1064 nm, and 1220 nm using lasers
at these discrete wavelengths.

Propagation losses measured near the signal and pump
wavelengths are shown in FIG. 19. We fit the loss spectrum
to a linear combination of a Lorentzian function and A~* to
exemplify two separate spectral trends: the absorption fea-
ture at 950 nm and the scattering loss. The waveguide loss
and facet coupling for both the pump and signal modes are
characterized from transmission measurements of wave-
guides with different lengths on a single chip. Since bends
are used to create the varying waveguide lengths, a second
test-structure is used to verify that the bend loss are negli-
gible. Pump light coupling loss at the input is (5.0+0.5) dB
and signal light coupling loss at the output is (12.105) dB.
The increased coupling loss compared to the simulation is
likely caused by residue from the die-release process,
observed, in SEM imaging. Additionally, the signal light
suffers from increased coupling loss due to the higher order

modes in the output tensed fiber. The propagation loss forthe 3

pump mode is (1.50.2) dB/om and for the signal mode is
(16.8+0.8) dB/om.

Conversion efficiency spectra are shown in FIG. 20(a) for
a length of 2.9 mm and a range of temperatures. The peak
efficiency is (39.5+7.9) W-' at 0.15° C. and decreases to
(28.5+5.7) W"! at 60° C. The uncertainty is calculated from
the coupling loss variation across the chip. The change in
peak SHG wavelength with temperature is 0.236 nm/? C., as.
shown in FIG. 20(6). The wavelength uncertainty arises
from mode-hopping in the laser, and it is calculated from
repeated measurements. We also measured SHG at 25° C,
from devices with lengths of 6.3 mm and 9.7 mm, but the
efficiency decreases to (31.4+6.2) W~' and (15.2#3.0) W7!,
respectively. The complete SHG spectrum is shown in FIG.
20(c) at room temperature (~22° C.), to show greater detail
than the temperature controlled measurements in FIG. 20(a).

Phase errors resulting from thickness non-uniformity
likely limit the nonlinear conversion for the longer wave-
guides that produce lower conversion efficiency. This is
evident from the SHG spectral shape from FIG. 20(c) that
does not show a clean sinc” function. By extrapolating the
uniformity measurement from across the wafer to within the
length of the SHG waveguide, we estimate that the average
GaAs thickness varies by up to ~20 pm. Compared to Ah
from FIG. 16 of ~7 pm, this thickness variation is very
significant. The average width of the waveguide may not
vary significantly compared to Aw of ~I nm. The 2.9 mm
long device may be more uniform than the maximum
variation of ~20 pm because it was located close to the
center of the wafer, where the thickness variation is minimal.

Reduction, of the waveguide loss at the signal wavelength
involves minimizing As—As bonds at the GaAs surface at

2

40

&

26
two different steps in the fabrication process. First, the
bonding surface of the GaAs waveguide layer must be
allowed to form a native oxide after the MBE growth, which
is not removed before bonding. This direct, bond between
the native GaAs-oxide surface and the SiO, surface is
optimal to reduce the As—As bonds. The second step is the
surface treatment of the top and sidewall of the GaAs
waveguide. Similarly, a native oxide is preferred, but only-
after an HCI strip of the surface reconstruction layer (the
amorphous surface layer naturally formed in air). This
allows for a native oxide to grow with minimal As—As
bonds. Propagation loss of the signal is (20.4#1.4) dB/cem
when the native oxide is stripped with HCI before bonding,

s and the waveguides are stripped with HCl after etching.

When the bonding is performed with the native GaAs oxide,
the loss of the signal is (34.6+3.7) dB/cm before treating the
waveguides with HCl.

As indicated from the propagation loss spectrum, this
device has some As—As bonds, and the loss from this defect
state can still be improved. A process to further reduce
As—aAs bonds is to use atomic layer deposition (ALD) to
passivate the surface. However, attempts with ALD Al,O,
passivation produced higher loss than the current process

5 Also, the loss at the pump wavelength indicates that our

sidewall roughness may have a significant contribution.
Further optimization of lithography and etching process may
reduce the loss by decreasing scattering for both the signal
and pump wavelengths.

At the elevated temperatures the decreased SHG conver-
sion efficiency may be due to an increase in propagation
losses. Free-carrier absorption in the GaAs for the signal is
expected to increase with temperature, and the peak wave-
length of the As—As defect state is strongly temperature
dependent. The conversion efficiency does not degrade after
the thermal cycling between 15° C. and 60° C., so the loss
is not a permanent effect. Regardless, we find that the
conversion efficiency is consistent from 15° C. to 40° C. An
f-2f self-referenced comb can reasonably be stabilized in
this temperature range.

Similar SHG results were measured immediately follow-
ing and several months after the fabrication, showing no
measurable degradation in performance. The peak effi-
ciency, bandwidth, and operation wavelength did not vary

5 more than our measurement error within this time frame.

These results show that the SHG performance is not affected
by oxidation or other chemical reactions with the GaAs.
A 76 mm wafer-scale GaAs-on-insulator platform was
made as an SHG device compatible with heterogeneous
integration on a SiN PIC for f-2f self-referencing and optical
frequency synthesis. SHG conversion efficiency of 40 W-!
(470 W-'cm?) in a 0.5 nm bandwidth is demonstrated from
a single-pass device because of improvements made to the
propagation loss and the waveguide uniformity. Efficient

5 SHG operation is confirmed over a temperature range of 45°

C., With these conversion efficiency achievements, one can
make a fully-integrated chip-scale f-2f system. To improve
SHG conversion efficiency, quasi-phase-matching provides
optimization of the nonlinear mode-overlap and for imple-
menting greater phase-matching tolerance. Applying orien-
tation patterning to the GaAs platform is expected to pro-
duce even higher SHG conversion efficiencies, exceeding
100 W-' with a broader bandwidth of >10* GHz.

While one or more embodiments have been shown and

5 described, modifications and substitutions may be made

thereto without departing from the spirit and scope of the
invention. Accordingly, itis to be understood that the present


U.S. Patent Aug. 10, 2021 Sheet 19 of 20 US 11,086,193 B1

nN
o

os
on

Lorentzian ft

Loss (dB/crn)
3

“neon, hte oMbined fit
a~4 a eee

1000 1200 1400 1600 1800 2060
Wavelength (nm)

enn

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FIG. 19


US 11,086,193 BI

9

tinuum nonlinear optical member 205. Supercontinuum
input optical taper 203 and supercontinuum output optical
taper 204 guide light into and out of supercontinuum gen-
erator section 202. An angle between tapered walls of
supercontinuum input optical taper 203 or supercontinuum
output optical taper 204 and a center axis of supercontinuum
generator section 202 independently can be from 0° to 150°,
specifically from 45° to 120°, and more specifically from
70° to 110°.

Second harmonic generator section 208 includes second
harmonic input optical taper 209 and second harmonic
output optical taper 210 between which is interposed second
harmonic nonlinear optical member 211. Second harmonic
input optical taper 209 and second harmonic output optical
taper 210 guide light into and out of second harmonic
generator section 208. An angle between tapered walls of
second harmonic input optical taper 209 or second harmonic
nonlinear optical member 211 and a center axis of second
harmonic generator section 208 independently can be from
0° to 150°, specifically from 45° to 120°, and more specifi-
cally from 70° to 110°.

Supercontinuum nonlinear optical member 205 guides
light from supercontinuum input optical taper 203 to super-
continuum output optical taper 204 and produces supercon-
tinuum light 207 from input light 206. Production of super-
continuum light 207 can occur photophysically through
supercontinuum nonlinear optical member 205 and a wave-
length under which supercontinuum light 207 is produced
from input light 206 can be selected by a physical size and
geometry of supercontinuum nonlinear optical member 205.
In an embodiment, a width of supercontinuum nonlinear
optical member 205 from supercontinuum input optical
taper 203 to supercontinuum output optical taper 204 can

vary in a continuous (panel B of FIG. 1) or undulating 3

contoured shape (as shown in panels C and D of FIG. 1) as.
viewed along Z-axis shown in the plan views of FIG. 1

Second harmonic nonlinear optical member 211 guides
light from second harmonic input optical taper 209 to second
harmonic output optical taper 210 and produces second
harmonic light 212 from supercontinuum light 207. Produc-
tion of second harmonic light 212 can occur photophysically
through second harmonic nonlinear optical member 211 and
a wavelength under which second harmonic light 212 is
produced from supercontinuum light 207 can be selected by
a physical size and geometry of second harmonic nonlinear
optical member 211

Input light 206 can have a wavelength from 1100 nm to
16000 nm, specifically from 1200 nm to 5000 nm, and more
specifically from 1250 nm to 2500 nm. Supercontinuum
light 207 produced from input light 206 can have a wave-
length from 600 nm to 16000 nm, specifically from 600 nm
to 6000 nm, and more specifically from 650 nm to 3000 nm.
Second harmonic nonlinear optical member 211 produced
from supercontinuum light 207 can have a wavelength from
600 nm to 8000 nm, specifically from 600 nm to 6000 nm,
and more specifically from 650 nm to 3000 nm. It is
contemplated that output light 213 propagates from self-
referencing nonlinear frequency converting photonic wave-
guide 200 and includes input light 206, supercontinuam
light 207, and second harmonic light 212. A pulse width of
light (206, 207, 212, 213) independently can be from 2 fs to
100 ps, specifically from 10 fs to 10 ps and more specifically
from 50 fs to 5 ps. An average power of light (206, 207, 212,
213) independently can be from 0.01 mW to 200 mW,
specifically from 0.05 mW to 50 mW, and more specifically
from 0.1 mW to 10 mW.

30

40

&

10

In an embodiment, with reference to FIG. 5, self-refer-
encing nonlinear frequency converting photonic waveguide
200 includes passivation layer 230 disposed on supercon-
tinuum generator section 202 and second harmonic genera-
tor section 208. Passivation layer 230 provides passivation
to self-referencing nonlinear frequency converting photonic
waveguide 200 from second dielectric medium 215 and can
include silicon dioxide, aluminum oxide, aluminum nitride,
hafnium oxide, silicon nitride, titanium oxide, tantalum
pentoxide, calcium fluoride, and the like. A thickness of
passivation layer 230 can be from 1 nm to | jum, specifically
from 1 nm to 100 nm, and more specifically from 2 nm to
10 nm.

Second dielectric medium 215 provides physical encap-
sulation or additional surface passivation and can include
silicon dioxide, aluminum oxide, aluminum nitride, hafnium
oxide, silicon nitride, titanium oxide, tantalum pentoxide,
calcium fluoride, and the like. A thickness of second dielec-
tric medium 215 can be from 1 nm to 1 um, specifically from
100 nm to 5000 nm, and more specifically from 200 nm to
3000 nm.

In an embodiment, with reference to FIG. 7, laser source
216 produces input light 206. Exemplary laser sources 216
include semiconductor diode lasers, fiber lasers, solid-state
lasers, and the like. A repetition rate of laser source 216 can
be from 10 KHz to 1 THz specifically from 1 MHz to 100
GHz, and more specifically from 10 MHz to 10 GHz.

Operation of laser source 216 is controlled by feedback
signal 217 that can include current, voltage, or light. Control
occurs by adjusting the cavity length through temperature
tuning, physical compression or expansion of the cavity’s
total length, or tuning of the refractive index of the media
within the laser cavity.

Output light 213 produced by self-referencing nonlinear
frequency converting photonic waveguide 200 is received
by wavelength splitter 218. Wavelength splitter 218 can
include chip-integrated elements or fiber-integrated compo-
nents to produce broadened spectrum 219 and combination
signal 222.

Broadened spectrum 219 includes some of the spectrum
generated by supercontinuum generation, in an absence of
less than 50% of the optical power contained in the disper-
sive wave of the supercontinuum spectrum and the light
generated in the second harmonic. Spectrum 219 can be used
in various articles such as a remote sensing device, a
chemical analysis device, and the like, among other appli-
cations.

Combination signal 222 can include light produced super-
continuum and second harmonic generation and can include
discrete optical frequency tones, with a frequency from the
supercontinuum-broadened and a frequency from second-
harmonic spectra, separated from each other by a small
frequency, for example from 1 Hz to 100 GHz, more

5 specifically from 1 Hz to 200 MHz.

Photodetector 223 receives combination signal 222 and
produces photosignal 224 therefrom. Photodetector 223 can
include a photo-sensing element, an optical window, control
or interface electronics, temperature control subsystems, and
the like. Exemplary photodetectors 223 include semicon-
ductor photodiodes, plasmonic photoreceivers, and the like.

Photosignal 224 includes an electrical beat-note including
a sinusoidal waveform centered at a frequency correspond-
ing to the frequency-difference between the adjacent optical

5 tones from the supercontinuum-broadened and second-har-

monic spectra, respectively. Spectrum analyzer 225 receives
photosignal 224 and produces analyzer signal 226 there-


US 11,086,193 BI

1
SELF-REFERENCING NONLINEAR
FREQUENCY CONVERTING PHOTONIC
WAVEGUIDE AND SELF-REFERENCING
NONLINEAR FREQUENCY CONVERSION

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH

This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce and under the DARPA SCOUT and DARPA
MTO DODOS programs. The Government has certain rights
in the invention. Licensing inquiries may be directed to the
Technology Partnerships Office, NIST, Gaithersburg, Md.,
20899; voice (301) 975-2573; email tpo@nist.gov; refer-
ence U.S. Pat. No. 17/044,853

BRIEF DESCRIPTION

Disclosed is a self-referencing nonlinear frequency con-
verting photonic waveguide for self-referencing nonlinear
frequency conversion, the self-referencing nonlinear fre-
quency converting photonic waveguide comprising: a sub-
strate; a dielectric layer disposed on the substrate with an
input terminus at which input light propagates toward an
output terminus opposingly disposed from the input termi-
nus; a supercontinuum generator section disposed at the
input terminus on the dielectric layer and comprising: Al,-

Ga, As_; a supercontinuum input optical taper disposed at the 3

input terminus and that receives input light, the supercon-
tinuum input optical taper tapering toward the input termi-
nus, a supercontinuum nonlinear optical member in optical
communication with the supercontinuum input optical taper

and that receives the input light from the supercontinuum 3

input optical taper and produces supercontinuum light from
the input light, the supercontinuum light spanning an optical
octave with respect to the input light; and a supercontinuum
output optical taper in optical communication with the
supercontinuum nonlinear optical member and that receives
the supercontinuum light and the input light from the
supercontinuum nonlinear optical member and propagates
the supercontinuum light and the input light from the
supercontinuum generator section, the supercontinuum out-
put, optical taper tapering away from the input terminus, a
first height H1 relative to a surface of the dielectric layer on
which the supercontinuum generator section is disposed; a
first width W1 of supercontinuum nonlinear optical member
orthogonal to a direction propagation of supercontinuum
light in supercontinuum generator section; and a second
harmonic generator section disposed on the supercontinuum
output optical taper and the dielectric layer and that extends
on the dielectric layer from the supercontinuum output
optical taper towards the output terminus and comprising:
Al,Ga, As, that provides birefringent modal phase matching,
a second harmonic input optical taper disposed on the
supercontinuum output optical taper and that receives the
supercontinuum light and the input light from the supercon-
tinuum output optical taper, the second harmonic input
optical taper tapering toward the supercontinuum output
optical taper; a second harmonic nonlinear optical member
in optical communication with the second harmonic input
optical taper and that receives the supercontinuum light and
the input light from the second harmonic input optical taper
and produces second harmonic light from the supercon-
tinuum light and the input light, the second harmonic light
comprising a second harmonic of the supercontinuum light,

S

20

2

40

&

2

a second harmonic output optical taper disposed at the
output terminus in optical communication with the second
harmonic nonlinear optical member and that receives the
second harmonic light, the supercontinuum light, and the
input light from the second harmonic nonlinear optical
member and co-propagates the second harmonic light, the
supercontinuum light, and the input light from the second
harmonic generator section toward the output terminus as
output light, the second harmonic output optical taper taper-
ing away from the second harmonic nonlinear optical mem-
ber toward the output terminus, a second height H? relative
to the surface of the dielectric layer on which the second
harmonic generator section is disposed; a second width W2
of supercontinuum nonlinear optical member orthogonal to

5 a direction propagation of second harmonic light in the

second harmonic generator section, such that the first height
H1 is greater than or equal to the second height H2, and the
first width W1 is less than or equal to second width W2.
Disclosed is a detector for detecting a frequency of a
carrier envelope offset, the detector comprising: a self-
referencing nonlinear frequency converting photonic wave-
guide comprising: a substrate; a dielectric layer disposed on
the substrate with an input terminus at which input light
propagates toward an output terminus opposingly disposed

3 from the input terminus; a supercontinuum generator section

disposed at the input terminus on the dielectric layer and
comprising: Al,Ga,As.; a supercontinuum input optical
taper disposed at the input terminus and that receives input
light, the supercontinuum input optical taper tapering toward
the input terminus; a supercontinuum nonlinear optical
member in optical communication with the supercontinuum
input optical taper and that receives the input light from the
supercontinuum input optical taper and produces supercon-
tinuum light from the input light, the supercontinuum light
spanning an optical octave with respect to the input light;
and a supercontinuum output optical taper in optical com-
munication with the supercontinuum nonlinear optical mem-
ber and that receives the supercontinuum light and the input
light from the supercontinuum nonlinear optical member
and propagates the supercontinuum light and the input light
from the supercontinuum generator section, the supercon-
tinum output optical taper tapering away from the input
terminus, a first height H1 relative to a surface of the
dielectric layer on which the supercontinuum generator
section is disposed; a first width W1 of supercontinuum
nonlinear optical member orthogonal to a direction propa-
gation of supercontinuum light in supercontinuum generator
section: and a second harmonic generator section disposed
on the supercontinuum output optical taper and the dielectric
layer and that extends on the dielectric layer from the
supercontinuum output optical taper towards the output
terminus and comprising: Al,Ga,As, that provides birefrin-
gent modal phase matching; a second harmonic input optical
taper disposed on the supercontinuum output optical taper
and that receives the supercontinuum light and the input
light from the supercontinuum output optical taper, the
second harmonic input optical taper tapering toward the
supercontinuum output optical taper; a second harmonic
nonlinear optical member in optical communication with the
second harmonic input optical taper and that receives the
supercontinuum light and the input light from the second
harmonic input optical taper and produces second harmonic
light from the supercontinuum light and the input light, the
second harmonic light comprising a second harmonic of the
supercontinuum light; a second harmonic output optical
taper disposed at the output terminus in optical communi-
cation with the second harmonic nonlinear optical member


US 11,086,193 BI

27

invention has been described by way of illustrations and not
limitation, Embodiments herein can be used independently
or can be combined.

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each
other. The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise
stated or contextually inapplicable, all percentages, when
expressing a quantity, are weight percentages. The
suffix “(s)” as used herein is intended to include both the
singular and the plural of the term that it modifies, thereby
including at least one of that term (e.g., the colorant(s)
includes at least one colorants). “Optional” or “optionally”
means that the subsequently described event or circumstance
can or cannot occur, and that the description includes
instances where the event occurs and instances where it does
not. As used herein, “combination” is inclusive of blends,
mixtures, alloys, reaction products, and the like.

As used herein, “a combination thereof” refers to a
combination comprising at least one of the named constitu-
ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (espe-
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-

text. “Or” means “and/or” It should further be noted that the 3

terms “first,” “second,” “primary,” “secondary,” and the like
herein do not denote any order, quantity, or importance, but
rather are used to distinguish one element from another. The
modifier “about” used in connection with a quantity is

inclusive of the stated value and has the meaning dictated by 3

the context (e.g., it includes the degree of error associated
with measurement of the particular quantity). The conjunc-
tion “or” is used to link objects of a list or alternatives and
is not disjunctive; rather the elements can be used separately
or can be combined together under appropriate circum-
stances.

What is claimed is:

1. A self-referencing nonlinear frequency converting pho-

tonic waveguide for self-referencing nonlinear frequency
conversion, the self-referencing nonlinear frequency con-
verting photonic waveguide comprising:

a substrate;

a dielectric layer disposed on the substrate with an input
terminus at which input light propagates toward an
output terminus opposingly disposed from the input
terminus,

a supercontinuum generator section disposed at the input
terminus on the dielectric layer and comprising:
Al,Ga,As.;

a supercontinuum input optical taper disposed at the
input terminus and that receives input light, the
supercontinuum input optical taper tapering toward
the input terminus:

a supercontinuum nonlinear optical member in optical
communication with the supercontinuum input opti-
cal taper and that receives the input light from the
supercontinuum input optical taper and produces
supercontinuum light from the input light, the super-
continuum light spanning an optical octave with
respect to the input light; and

a supercontinuum output optical taper in optical com-
munication with the supercontinuum nonlinear opti-

2

40

60

6:

28

cal member and that receives the supercontinuum
light and the input light from the supercontinuum
nonlinear optical member and propagates the super-
continuum light and the input light from the super-
continuum generator section, the supercontinuum
output optical taper tapering away from the input
terminus,

a first height H1 relative to a surface of the dielectric
layer on which the supercontinuum generator section
is disposed;

a first width W1 of supercontinuum nonlinear optical
member orthogonal to a direction propagation of
supercontinuum light in supercontinuum generator
section; and

second harmonic generator section disposed on the

supercontinuum output optical taper and the dielectric

layer and that extends on the dielectric layer from the
supercontinuum output optical taper towards the output
terminus and comprising:

Al,Ga,As, that provides birefringent modal phase
matching;

a second harmonic input optical taper disposed on the
supercontinuum output optical taper and that
receives the supercontinuum light and the input light
from the supercontinuum output optical taper, the
second harmonic input optical taper tapering toward
the supercontinuum output optical taper;

a second harmonic nonlinear optical member in optical
communication with the second harmonic input opti-
cal taper and that receives the supercontinuum light
and the input light from the second harmonic input
optical taper and produces second harmonic light
from the supercontinuum light and the input light,
the second harmonic light comprising a second har-
monic of the supercontinuum light;

a second harmonic output optical taper disposed at the

output terminus in optical communication with the

second harmonic nonlinear optical member and that
receives the second harmonic light, the supercon-
tinuum light, and the input light from the second
harmonic nonlinear optical member and co-propa-
gates the second harmonic light, the supercontinuum
light, and the input light from the second harmonic
generator section toward the output terminus as
output light, the second harmonic output optical
taper tapering away from the second harmonic non-
linear optical member toward the output terminus;
second height H2 relative to the surface of the
dielectric layer on which the second harmonic gen-
erator section is disposed;

a second width W2 of supercontinuum nonlinear opti-
cal member orthogonal to a direction propagation of
second harmonic light in the second harmonic gen-
erator section,

such that the first height H1 is greater than or equal to the

second height H2, and the first width W1 is less than or

equal to second width W2.

2. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, further comprising a passi-
vation layer disposed on the supercontinuum generator
section.

3. The self-referencing nonlinear frequency converting
photonic waveguide of claim 1, further comprising a second
dielectric medium disposed on the supercontinuum genera-
tor section.

.

.


U.S. Patent Aug. 10, 2021 Sheet 16 of 20 US 11,086,193 B1

AR fom)

9) ; ; 4 : ; : &
140 142 #144 «#146 #4148 #150 182 154
Height (nm)

FIG. 16


U.S. Patent Aug. 10, 2021 Sheet 8 of 20 US 11,086,193 B1

Power on system

Transmit pulsed
light source to
nanophotonic

AlGaAs chip

Receive

supercomtingum
and secand-

harmonic signals

Detect F2f
beatnote

FIG. 8


US 11,086,193 BI

21

operation is demonstrated over a temperature range of 45°C.
with a slope of 0.24 nm/° C. Wafer-bonding between GaAs
and SiO, is optimized to minimize waveguide loss, and the
devices are fabricated on 76 mm wafers with high unifor-
mity. This device fully integrated self-referenced frequency
combs and high-rate entangled photon pair generation.

Optical frequency conversion uses nonlinear interactions
of light with matter to generate new wavelengths of light.
Recent efforts have improved conversion efficiencies and
broadened the application of this technology by using nano-
photonic waveguides. Second harmonic generation (SHG) is
one nonlinear process of significant interest because it is
critical for stabilizing optical frequency combs. In SHG,
input pump light is converted to a signal with twice the
frequency. It is used to self-reference a frequency comb by
comparing and controlling the difference between two sec-
tions of the comb spectra that are separated by an octave.
This is referred to as f-2f self-referencing, and it produces
low-noise optical frequency combs used for optical clocks,
optical frequency synthesizers, low-phase-noise microwave
generation, and molecular sensing

Increasing the application space for stabilized frequency
combs can involve reducing cost, size, weight, or power
consumption while maintaining performance metrics. Chip-
scale integration is an attractive way to achieve these goals,
particularly using heterogeneous integration to combine a
suite of different materials into a compact package. A
promising approach to develop a chip-scale stabilized fre-
quency comb is to pump a SiN microresonator with, a

continuous-wave (CW) near-IR laser. This design is scalable 3

to large-volume and high-yield manufacturing to reduce
cost. However, exceptionally high nonlinear conversion
efficiencies are required for this fully integrated system. In
particular, avoiding the use of amplifiers for the microreso-

nator pump laser and the SHG pump light will be a signifi- 3

cant advancement. Given the limitations presented by the
SiN microresonator quality factor that restricts the pump
power and by the detector noise, extremely high SHG
efficiency is necessary to achieve a suflicient signal-to-noise
ratio,

A comparison of the highest SHG conversion efficiencies
with the corresponding signal bandwidth from different
waveguide platforms is shown in FIG. 14 for the non-
depleted pump regime. The pump wavelength is also indi-
cated since the conversion efficiency is generally higher for
shorter wavelengths with a given platform. Neglecting the
effects of non-uniformity and loss, conversion efficiency is
expected to increase inversely proportional to the square of
the bandwidth. Larger SHG bandwidths are especially
important for applications where the pump is either broad or
consists of multiple narrow-linewidth modes that reside
within the SHG bandwidth. In certain cases, one narrow-
linewidth pump mode is relevant so larger SHG bandwidths
may not be helpful as long as the pump mode can be aligned
to the SHG conversion band. Another aspect to consider in
this comparison is that SHG is most applicable as a com-
ponent of a system. Therefore, relevant SHG technologies
should either support versatile and low-loss coupling tech-
niques or be compatible with heterogeneous integration. For
example, to add SHG functionality to photonic integrated
circuit (PIC) based on mature and sealable Si photonics, it
is advantageous to either deposit, grow, or bond the nonlin-
ear material to the Si substrate.

Integrated f-2f self-referencing without an optical ampli-
fier or an assist-laser requires an SHG conversion efficiency
on the order of 40 W~". At this level, 5 uW of CW pump light
(the typical power from a single comb line) can be converted

3

22

to 1 nW of signal light. To our knowledge, no single pass
device has previously demonstrated this. While some reso-
nant devices have exceeded this efficiency, the narrow
bandwidth and sensitive phase-matching condition makes
them impractical for a fully integrated f-2f self-referencing
system. These resonant devices are better suited for para-
metric down-conversion applications with narrow band-
widths and squeezed light generation. Furthermore. produc-
ing such systems on a large scale requires a high-yield and
reproducible fabrication. An SHG device herein has high
efficiency and bandwidth for an integrated f-2f self-refer-
enced comb. The high second-order nonlinearity of GaAs
paired with the high index contrast and uniformity of
integrating GaAs-based waveguides on Si has enabled this
advancement. The device permits birefringent modal phase-
matching in a GaAs waveguide to produce SHG from a 2.0
pump with an efficiency of 40 W~! and a signal bandwidth
of 0.5 nm. The device is fabricated at the 76 mm (3-inch)
wafer scale and is compatible with integration on the SiN
platform used for the f-2f self-referencing.

Efficient SHG conversion with modal phase-matching is
provided by the self-referencing nonlinear frequency con-
verting photonic waveguide described here that can be made
from processing techniques for wafer-scale fabrication. The
conversion efficiency is characterized to determine the peak
efficiency, bandwidth, and temperature dependence.

To design the SHG waveguide, select a phase-matched
geometry that maximizes the conversion efficiency, 7,
defined as:

w

wherein P,,, is the signal power, P,, is the pump power, L is
the waveguide length, w is the angular frequency of the
pump, €0 is the permittivity of free space, and c is the speed
of light in vacuum. The effective refractive indices of the
pump and signal optical modes are n,, and na,,, respectively.
The & term in Eq. (1) accounts for phase-matching and the
propagation loss: Note that in a typographical error exists: in
Eq. (3), the a appearing in the cosine term should be
replaced with A.

+B 2 (Qa)
é=— elruitourl,
a? + APL /29"
wherein
Arsin h(AaL/2)}cos(ABL/2), (2b)
B=cos h(AaL./2)sin(ABL/2). (Qc)

Parameters o.,, and @,,, are the propagation loss coefli-
cients for the pump and signal. The phase-mismatch, Ap,
and the loss-mismatch, Ac, are defined as AB=B20-2Bo0=
Qe/c)(n2e-nw) and Aa=a20/2-cu. A nonlinear coupling
parameter, x, in Eq. (1) accounts for two effects: the mode
overlap between the pump and signal light and the interac-
tion between the optical modes and the nonlinear optical
susceptibility of the waveguide. It is defined as:

fo)

(fferlBvul?axdyy’ f f2lEy aul dxdy


US 11,086,193 BI

15

intrinsic second-order optical nonlinearities for highly effi-
cient frequency conversion and electro-optic modulation.

Alternatively, group III-V materials possess many desir-
able properties for multifunctional integrated photonic sys-
tems, including a high refractive index, strong second- and
third-order optical nonlinearities, and wide optical transpar-
ency windows into the LWIR. A practical advantage of these
materials is the ability to grow a chemically selective etch
stop underneath a high-quality epitaxial device (donor) film,
enabling wafer or chip-bonding film transfer techniques for
heterogeneous integration. This has enabled high-index-
contrast III-V waveguides on other substrates such as oxi-
dized silicon and sapphire. However, to take full advantage
of the broad transparency window supported by III-V semi-
conductors in a high-index contrast environment, it is nec-
essary to pursue alternative geometries such as air-clad
suspended waveguides. Materials can form surface oxide
layers that also introduce absorption. Undercut etching has
been used to suspend GaAs waveguides engineered for
mid-IR difference frequency generation. While this repre-
sents a promising step in the development of nonlinear
mid-IR photonics with III-V materials, many issues remain,
such as the propagation loss in the mid-IR region, atmo-
spheric stability, coupling losses, and amenability to wafer-
scale production. Without thick cladding layers protecting
the surface, suspended III-V waveguides become suscep-
tible to several strong mid-IR and LWIR absorption bands
from surface oxidation.

Air-clad mid-IR waveguides include suspended AlGaAs

on silicon, which is shown schematically in FIG. 9. We 3

employ direct-bonded membranes, which provide superior
mechanical stability and design flexibility thanks to the free
choice of geometry and absence of stress-inducing perfora-
tions. The issue of surface-oxide losses is addressed by
passivating the AlGaAs surfaces with ultrathin films of
sputtered silicon nitride (SiN), bringing the loss of wave-
guides from more than 100 dB/cm (without passivation) to
<2.5 dB/em (after passivation) through most of the mid-IR.
The fabrication technique described in this work can also be
applied to other III-V materials such as GaAs, GaP and GaN.
In the case of GaP and GaN, the larger bandgaps could
enable even wider transparency windows. However, due to
the relative maturity of its device processing and material
growth, we chose AlGaAs as the suspended material here.
‘A fabrication approach to realize Aly ssGao g¢As (bandgap
of A=681) photonic waveguides on a conventional silicon 76
mm-diameter wafer (FIG. 9). Design and characterization of
several passive devices include microring resonators, com-
pact waveguide bends, inverted taper edge couplers, and
multimode interferometer (MMI) 1x2 splitter junctions.
Dispersion-engineered waveguides achieve second-har-
monic generation (SHG) and supercontinuum generation
from femtosecond laser sources at low pulse energies, and
we compare the results for near-IR and mid-IR pumping.
A schematic representation of the fabrication process is
provided in FIG. 9(@). Fabrication runs were conducted with
variations on the main fabrication flow. The illustrated
process captures the essential details of the best-performing
variation, on which most of the results of this work are
focused. The fabrication process can include cleaning a
silicon handle wafer; etching trench features where wave-
guides will be formed; bonding an epitaxial III-V wafer to
the handle (a SiN passivation layer has been applied prior to
the bond), chemically removing the III-V wafer substrate
(GaAs) and etch-stop layer (Alp sGag.As); electron-beam
lithography and plasma etching of the ridge waveguide
features over the trenched areas, and final oxide stripping

S

40

&

16

and top surface passivating with SiN. Individual dies were
released from the wafer simultaneously using deep reactive
ion etching (DRIB) through the silicon handle wafer. The
dies were then annealed at 300° C. in a nitrogen environ-
ment, which reduced losses from N—H bond absorption.
The devices have a nominal core thickness of 540 nm and a
slab thickness of 90 nm [FIG. 9(4)]. The trenches underneath
the waveguides are 6 fm deep and about 11 jum wide. The
SiN barriers on the bottom and top surfaces of the membrane
are 10 and 20 nm thick, respectively. No die-level processing
was necessary, enabling parallel fabrication of dozens of
chips without any polishing and cleaving, that might be
involved with sapphire or group III-V substrates. This is
provided as a benefit, of a silicon substrate.
High-performance passive devices are involved for inte-
grating multiple components on one chip, such as nonlinear
frequency converters, modulators, and detectors. The ability
to simultaneously realize precisely dispersion-engineered
waveguides with tight bending radii, small mode volumes,
and low coupling losses is not guaranteed in ail waveguide
platforms. To this end, we designed, fabricated, and tested
several key passive elements, including microring resona-
tors, bends, 1x2 MMI power splitters, and input/output
(edge) couplers. First, we consider the characterization of

s suspended AlGaAs microring resonators over a broad wave-

length range from the near- to mid-IR. We show that the
passivation approach was effective at suppressing the inher-
ently high absorption loss of bare AlGaAs surfaces, achiev-
ing losses below 2.5 dB/cm out to 4=4.6 jm. An analysis
focused on specific loss contributions in the mid-IR, includ-
ing surface states of unpassivated waveguides.

Microring resonators provide a convenient gauge for the
waveguiding performance of any integrated photonic plat-
form. Since we are interested in nonlinear applications
involving pump wavelengths across the near-IR and mid-IR,
we examined the performance of several resonator devices
at A=1.26, 1.59, 2.39, 3.64, and 4.56 jum. For all wave-
lengths, light was aligned to the quasi-transverse-electric
(TE) mode of the waveguides.

The results are collected in FIG. 10. The broadband
spectral dependence of propagation loss is shown in FIG.
10(d). The relevant device parameters (ring waveguide
width, ring radius, etc.) and measurement results are shown
for each resonance in FIGS. 10(e)-10(i) The resonances in

5 this figure each represent the highest quality factor (QQ)

observed for that wavelength point. The maximum loaded Q
of 8.8x10° is achieved in the 2-"m-wide ting device at
4=1592 nm. Factoring out the coupling loss of the ring
(conservatively assuming an under-coupled condition), this
corresponds to an intrinsic Q of 1.1x10°, giving a waveguide
propagation loss of 0.54 dB/cm. Going to longer wave-
lengths, we measured propagation losses of 0.45, 1.4, and
2.4 dB/em for 4=2.39, 3.64, and 4.56 yum, respectively. It can
be seen that the propagation loss exhibits a trough between

5 4=1500-2500 nm, with a sharply rising loss on the short-

wavelength side and a slow increase toward longer wave-
lengths. The blue-side increase can readily be attributed to
the well-known phenomenon of surface-state absorption
observed in GaAs/AlGaAs waveguides, which increases
sharply at wavelengths near A=1 jun. The fabrication process
was not optimized to enhance the performance in this region,
but specific treatments are possible if low-loss operation is
here. The loss observed from A=1500-2500 nm is mostly
scattering loss. The trend of ring waveguide width versus

5 loss is plotted in FIG. 10(c). It can be seen that the

propagation loss does not bottom out even at the widest
width considered, showing that sidewall scattering is still a


U.S. Patent Aug. 10, 2021 Sheet 5 of 20 US 11,086,193 B1

1.00

:

i

' 207, 208 +215

4 t

' 230 :

( ;
20]

FIG. 5


U.S. Patent Aug. 10, 2021 Sheet 4 of 20 US 11,086,193 B1

(Ay 200 205
Co
203+ aot
202

209 4) 2d ml

( ( | _

Q t T
214 8 ay?
rat

Ly Lai

D0 TF
@ Re bats
2
wo a i
{ —-
}
208 i
7 224
20
+.

ty Lal

FIG. 4



U.S. Patent Aug. 10, 2021 Sheet 17 of 20 US 11,086,193 B1

Si substrate

PIG, 17


U.S. Patent Aug. 10, 2021 Sheet 7 of 20 US 11,086,193 B1

au
231 prsseeedew meng
26
P27
27 | 1s row
aa) i wt

FIG. 7


US 11,086,193 BI

5

MMIs. Bottom inset of (e): top view of the simulated optical
intensity of the designed MMI splitter;

FIG. 12 shows supercontinuum generation from a 1560
nm pump. (a) Measured spectra for different waveguide-
coupled pulse energies. Octave-spanning bandwidth is high-
lighted for the case of 3.4 pJ. Trace-to-trace offset is 20 dB.
(b) Polarized supercontinuum output of a similar waveguide
for the TE- and TM-pass cases, showing suppression of the
second-harmonic peak for TE pass; (c) simulated waveguide
GVD; inset, intensity profile for the mode under consider-
ation;

FIG. 13 shows (a) supercontinuum generation from a
3060 nm pump; experimentally measured spectra for differ-
ent waveguide-coupled pulse energies (solid lines), and
simulated spectrum at 45 pI waveguide-coupled pulse
energy (dotted line); trace-to-trace offset, 30 dB; (b) super
continuum output for various total waveguide lengths at 67
pJ pulse energy; trace-to-trace offset, 20 dB; inset, optical
micrograph of a paper-clip structure used for length varia-
tions. Dashed gray lines in (a, b) indicate approximate noise
floor for each trace. (c) Simulated waveguide GVD; inset,
intensity profile for the mode under consideration;

FIG. 14 shows highest SHG conversion efficiencies dem-
onstrated from different waveguide material platforms for
resonant (circles) and single-pass (triangles) devices. The
signal bandwidth is defined by the FWHM. References:
AlGaAs-OI chip, AIN rings, GaAs-Ol chip, GaAs-OI ring,
GaP PhC, LNOI, LNOI ring, LN RPE, OP-AlGaAs, ox-
AlGaAs, PP-GaN, PPKTP, SiN/LNOT,

FIG. 15 shows (a) effective indices of the signal (solid 3

lines) and pump (dashed lines) as a function of waveguide
width. Each curve represents a different waveguide height,
from 140 nm with the lowest indices to 156 nm with the
highest indices, in steps of 2 nm between curves. (b) SHG

conversion efficiency (m) for a length of 2.9 mm plotted in 3

circles on the left axis as a function of waveguide height for
the perfectly phase-matched widths plotted in triangles on
the right axis;

FIG. 16 shows FWHM of the conversion efficiency ()
for variations in the width (Aw) shown in solid blue on the
left axis and the height (Ah) shown in dashed orange on the
right axis;

FIG. 17 shows (a) schematic diagram of the waveguide
cross-section. (b) Picture of the 76 mm wafer shows nearly
perfect yield of the transferred GaAs film. (c) SEM showing
the output waveguide facet;

FIG. 18 shows setup for SHG. The connections are
single-mode fibers and connections for polarization-main-
taining single-mode fibers. PC: polarization controller: Pol.:
fiber-based linear polarizer; PD: photodetector, WDM:
wavelength division multiplexer for splitting the 1 and 2
light:

FIG. 19 shows measured propagation loss showing the
trends of a Lorentzian, A~*, and a linear combination of both
(solid), and

FIG. 20 shows (a) SHG spectra for various temperatures,
limited by the temperature control setup. The standard error
is indicated by the shaded areas. (b) Peak SHG wavelength
for various temperatures with a linear fit of 0.236 nm/° C. (c)
The full spectrum of the SHG conversion efficiency at room
temperature (~22° C.), to show greater detail.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is
presented herein by way of exemplification and not limita-
tion.

S

2

40

&

35

6

It has been discovered that a self-referencing nonlinear
frequency converting photonic waveguide herein can be
used in an optical frequency comb system and includes
nano-scale photonic waveguides made of aluminum gallium
arsenide (Al,Ga,As.) that provided nonlinear frequency
conversion for f-to-2f self-referencing. Frequency accuracy
of a frequency-comb is provided by f-to-2f self-referencing,
wherein an optical octave is generated, and the low-fre-
quency range of the optical octave is frequency doubled and
compared to the high frequency range of the optical octave
to measure a frequency of a carrier envelope offset. In this
manner, the frequency comb links radio frequencies and
optical frequencies. In conventional frequency combs, pump
diode lasers make a high-power pulse that forms the optical

5 octave, and a waveguide doubler that include ppLN or

ppKTP perform frequency doubling. Nonlinear material
such as ppLN and ppKTP are expensive. Advantageously,
the self-referencing nonlinear frequency converting pho-
tonic waveguide includes Al,Ga,As_ and produces an optical
octave of light from Yooth the power of input light of
conventional devices and decreases cost of construction and
operation for frequency combs as compared with conven-
tional devices that use pump diodes. Beneficially, the Al,-
Ga,As. waveguide of the self-referencing nonlinear fre-

S quency converting photonic waveguide is a waveguide

doubler.

The self-referencing nonlinear frequency converting pho-
tonic waveguide includes a nanophotonic waveguide made
of Al,Ga,As, that is integrated on a substrate. A propagation
length of the waveguide is dispersion-engincered for super-
continuum generation from a pulsed pump laser, e.g., at
1560 nm, such that light is nonlinearly broadened to an
octave, e.g., from 900 nm to 1800. A width of the Al,Ga,As_
waveguide can be selectively tailored for cascaded second-
order nonlinearities. Another portion of a propagation length
of the waveguide produces second-harmonic generation by
modulating the structure to neutralize momentum mismatch
between fundamental light and second-harmonic light, e.g.
1800 nm fundamental light and 900 nm doubled light.
Form-birefringent phase matching or mode-shape modula-
tion that varies the width of the waveguide periodically can
provide supercontinuum generation or second harmonic
generation.

The waveguide can include edge couplers to optimize a

5 coupling input efficiency of input light and coupling output

efficiency for the second-harmonic light. Edge couplers can
include tapered nanophotonic waveguides to expand a mode
field diameter for coupling to tree-space or fiber-optic
modes. It is contemplated that minimization of absorption
losses at wavelengths from 800 nm to 1100 nm can include
a silicon substrate with a layer of silicon dioxide thereon
with an Al,Ga,As. waveguide having a thickness from 130
nm and 400 nm in an absence of material on top, referred to
as an air-top-cladding. The waveguide can include Al,-
Ga,As, surrounded by gas on both the top and bottom,
referred to as a suspended or fully air-clad device. The
waveguide can be tethered to the substrate by a layer of
Al,Ga, As. for index contrast, low coupling losses, and
optical transparency from visible to mid-infrared wave-
lengths.

Self-referencing nonlinear frequency converting photonic
waveguide 200 performs self-referencing nonlinear fre-
quency conversion. In an embodiment, with reference to
FIG. 1, FIG. 2, FIG. 3, and FIG. 4, self-referencing nonlinear

5 frequency converting photonic waveguide 200 includes:

substrate 214: dielectric layer 201 disposed on substrate 214
with input terminus 228 at which input light 206 propagates


a2 United States Patent

Chiles et al.

US011086193B1

US 11,086,193 B1
Aug. 10, 2021

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

(54)

(71)

(72)

(73)

(*)

(21)

(22)

GD

(52)

(58)

SELF-REFERENCING NONLINEAR
FREQUENCY CONVERTING PHOTONIC
WAVEGUIDE AND SELF-REFERENCING
NONLINEAR FREQUENCY CONVERSION

Applicant:

Inventors:

Assignee:

Government of the United States of
America, as represented by the
Secretary of Commerce, Gaithersburg,
MD (US)

Jeffrey Taylor Chiles, Boulder, CO
(US); Erie John Stanton, Boulder, CO
(US); fan Robert Coddington,
Boulder, CO (US)

GOVERNMENT OF THE UNITED
STATES OF AMERICA, AS
REPRESENTED BY THE
SECRETARY OF COMMERCE,
Gaithersburg, MD (US)

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

Appl. No.: 17/004,853

Filed: Aug. 27, 2020

Int. Cl.

GO2F 1/37 (2006.01)

GO2F 135 (2006.01)

GO2F 1/365 (2006.01)

US. Cl.

CPC G02F 1/37 (2013.01), G02F 1/3544

(2013.01); G02F 1/365 (2013.01); GO2F
1/3507 (2021.01)

Field of Classification Search

CPC

GO2F 1/3528

See application file for complete search history.

200

(56) References Cited

U.S. PATENT DOCUMENTS
10,566,765 B2* 2/2020 Kurezveil

10,797,468 B2* 10/2020 Kurezveil
2017/0317471 AL* 11/2017 Lor

HOIS 5/1032
HOIS 5/1067
G02B 6/124

OTHER PUBLICATIONS

I. Alamgir, M. H. M. Shamim, M. El Amraoui, Y. Messaddeq and
M. Rochette, “Supercontinuum Generation in Suspended Core
As2S3 Tapered Fiber,” 2020 IEEE Photonics Conference (IPC),
Vancouver, BC, Canada, 2020, pp. 1-2. (Year: 2020).*

Y. Zhang et al., “Supercontinuum Generation in Cascaded Photonic
Crystal Fiber Tapers,” 2019 18th International Conference on
Optical Communications and Networks (ICOCN), Huangshan, China,
2019, pp. 1-3. (Year: 2019).*

(Continued)

Primary Examiner — Rhonda S Peace
(74) Attorney, Agent, or Firm — Office of Chief Counsel
for National Institute of Standards and Technology

(57) ABSTRACT

A self-referencing nonlinear frequency converting photonic
waveguide includes: a supercontinuum input optical taper; a
supercontinuum nonlinear optical member that produces
supercontinuum light spanning an optical octave with
respect to input light; and a supercontinuum output optical
taper; a second harmonic input optical taper; a second
harmonic nonlinear optical member that receives the super-
continuum light and produces second harmonic light from
the supercontinuum light, the second harmonic light includ-
ing a second harmonic of the supercontinuum light; and a
second harmonic output optical taper that receives the
second harmonic light, the supercontinuum light, and the
input light and co-propagates the second harmonic light, the
supercontinuum light, and the input light from the second
harmonic generator section as output light.

19 Claims, 20 Drawing Sheets



U.S. Patent Aug. 10, 2021 Sheet 10 of 20 US 11,086,193 B1

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US 11,086,193 BI

19

referenced with no amplifiers following the oscillator, or
very high repetition-rate systems. Significantly stronger
SHG can readily be achieved in the future through the use
of quasi-phase-matching. The short-wavelength supercon-
tinuum could, also be enhanced via treatments to reduce the
short-wavelength losses from surface states on the AlGaAs
membrane, which we assume to be the current, limitation to
the bandwidth.

Consider broadening of pulsed, lasers in the mid-IR
region. Here, the pump was produced via difference fre-
quency generation of an amplified and broadened erbium
fiber comb output. The nominal pulse width was 85 fs, the
repetition rate was 100 MHz, and the center wavelength was
3.06 um. We focused on a waveguide geometry with a core
width of 2.15 jim, an effective mode area of 1.2 1m?, and a
nominal length of 2.3 mm, excluding tapered regions. The
GVD and intensity mode profile are depicted in FIG. 13(c),
showing a much flatter and broader region of anomalous
dispersion compared to the 1550 nm case (owing to the
strongly reduced bulk GVD contribution in the raid-IR for
AlGaAs). The waveguides were measured by coupling the
pump light in the TE-mode orientation and analyzing the
output spectrum in a Fourier-transform infrared spectrom-
eter (FTIR). The light was coupled out through an aspheric
lens aligned to collimate the long-wave side of the spectrum.
FIG. 13(a) shows the measured spectra at different wave-
guide-coupled pump pulse energies. At the lowest energy of
11 pJ, the pump exhibited no significant broadening. Super-
continuum generation occurred at 45 pJ, showing a smooth,

mostly unstructured spectrum spanning out to 6200 nm with 3

a small dispersive wave peak near 6000 nm. At higher pump
powers, the spectrum flattened out and broadened slightly.
We also investigated variations on the waveguide length in
FIG. 13(6). A fixed waveguide-coupled pulse energy of 67

pJ was used here. As expected, longer waveguides resulted 3

in the generation of more solitons, giving a more structured
spectrum as the pulse progressed to a final length of 10.5
mm.

Returning to FIG. 13(a), compare the experimental versus
simulated results for 45 pJ of waveguide-coupled pulse
energy. The simulations use an n of 0.9x107!7 m?/W and
a uniform propagation loss of 10 dB/cm. This loss can be
confined to a narrow peak near A=3 um from N—H bond
absorption. The material dispersion was modeled as n(2..x)
=Ngass(h)-0.45x for A>1.1 um, where ng,45 is the material
dispersion of GaAs, and xx is the mole fraction of aluminum.
for a given AlGaAs composition. The structure of the
simulated spectrum largely agrees with the experiment. The
simulation produces a dispersive wave near 4=6 um with a
smooth region connecting it to the pump. However, the
short-wavelength end of the spectrum has some dissimilar
features, such as the absence of a simulated dispersive wave
at 4=2.2 jum. The location of the dispersive wave is strongly
dependent on the zero crossing in the GVD curve, so even
a small deviation in the geometry or material dispersion
could result in significant changes to the spectrum. This
could be resolved with more precise measurements of the
AlGaAs core thickness, as well as direct measurements of
the refractive index of this particular AIGaAs composition in
the mid-IR using spectroscopic ellipsometry.

Larger broadening in the long-wave spectrum can occur
for waveguides with modified dispersion profiles, such as
deeper etches or slightly thicker core regions, which should
increase the bandwidth of the anomalous dispersion region.
For example, increasing the core thickness of the waveguide
by only 20 nm extends the anomalous GVD region to 2.7-5.1
jum instead of 2.9-4.8 um, producing a redshift by almost

20

600 nm in the long-wave dispersive wave. The thickness in
this work was chosen as a compromise enabling broadband
anomalous dispersion to be engineered from 4=1.5 to 4.8
yum, depending on the waveguide width. SHG was not
observed for 3060 nm pumping, most likely due to the
increased mode area.

The integrated photonic article included suspended
AlGaAs waveguides on silicon. By using directly bonded
AlGaAs membranes on pre-etched trenches in a silicon
substrate, we have overcome many obstacles to the adoption
of III-V materials for integrated photonics in the mid-IR.
With this approach, multifunctional devices leveraging x
and x nonlinearities can be fabricated reliably at the
wafer-scale with high coupling efficiency and low propaga-

5 tion losses. We show the all-around strengths of this plat-

form through a series of linear and nonlinear experiments
The relatively wide bandgap of this AlGaAs composition
has enabled us to use near-IR pumps to achieve SHG and
octave-spanning supercontinuum generation in dispersion-
engineered AlGaAs waveguides at remarkably low wave-
guide-coupled pulse energies of 3.4 pJ. We also realized
octave-spanning supercontinuum generation in the mid-IR,
from =2.3-6.5 um. High-quality microring resonators were
fabricated, and a loss of 0.45 dB/cm was realized at a
wavelength of 2.4 um. A minimum coupling loss of 3.0£0.1
dB/facet was observed at A=4.6 jum. Low-loss passive
elements, including compact waveguide bends (0.06 dB loss
per 90° bend with 10 um radius) and 1x2 MMI power
splitter junctions with only 6 um total length and 96#1%
power efficiency have also been demonstrated in the mid-IR.
Crucial to the practical development of this platform, we
characterized the performance of unpassivated AlGaAs
waveguides and showed that proper passivation is essential
to low-loss operation in the mid-IR. We developed SiN
passivation, allowing waveguides to be operated in air for
long time periods (most loss values were measured >2
months after fabrication). All of this has been achieved at the
wafer-scale with no required die-level processing, a valuable
practical advantage over native III-V platforms. The com-
bination of a wide optical transparency window, strong ¥°
and y® nonlinearities, and the bandgap of 681 nm may
enable ultra-eflicient frequency converters bridging the gap
from the near-IR to the mid-IR. Furthermore, the observed
ultralow thresholds for optical nonlinearities, combined with
the low propagation losses realized in this work, may point
to new opportunities for on-chip quantum photonics at 1550
nm and beyond. Suspended AlGaAs on silicon is a high-
performance platform for multifunctional integrated photon-
ics with both passive and nonlinear systems.

Example 2

Nonlinear frequency conversion plays a role in advancing
the functionality of next-generation optical systems. Por-
table metrology references and quantum networks will
demand highly efficient second-order nonlinear devices, and
the intense nonlinear interactions of nanophotonic wave-
guides can be leveraged to meet, these requirements. This
Example describes second harmonic generation (SHG) in
GaAs-on-insulator waveguides with unprecedented efli-
ciency of 40 W-! for a single-pass device. This result is
achieved by minimizing the propagation loss and optimizing
phase-matching. Surface-state absorption and design the
waveguide geometry for modal phase-matching with toler-
ance to fabrication variation occurred. A 2.0 um pump is
converted to a 1.0 um signal in a length of 2.9 mm with a
wide signal bandwidth of 148 GHz. Tunable and efficient


US 11,086,193 BI

17

significant contribution to the overall loss. FIG. 10(a) shows
evidence of sidewall corrugations resulting from roughness
in the lithography that is transferred directly to the device,
indicated by striations that are uniform along the vertical
axis, but varying in position in the width axis of the
waveguide. Multipass electron-beam lithography or resist
reflow can be employed to mitigate this effect. Ring reso-
nance data were taken more than two months after fabrica-
tion of the chips, proving the effectiveness of the passivation
layer at preventing losses from oxidation over time.

In order to achieve efficient frequency conversion with
integrated photonic devices, it is critical to have low-loss
and broadband, on- and off-chip couplers. We pursued
inverted taper couplers to meet these goals. The facets are
prepared in parallel via plasma etching. An optical micro-
graph of a typical edge coupler is shown in the lower inset
of FIG. 11(a). They employ two stages of tapering [upper
inset of FIG. 11(a)], which reduces the total length. We
consider several couplers with corresponding wavelengths
of interest to this project: 1.55, 2.3, and 4.6 jum. The results
are shown in FIG. 11(a). In many cases, the taper length did
not factor significantly into the performance, but it is
included for completeness. The lowest coupling loss of
3.0+0.1 dB is achieved at X=4.6 wm, where this value
corrects for the expected propagation loss of 0.2 dB in a 1
mm long chip at this wavelength. The contribution of
surface reflection to the loss of this taper tip was estimated
to be 1.2%. Experimental losses are reported with respect to
the reference transmission of light through both aspheric

lenses with no chip. The increase in loss toward shorter 3

wavelengths is expected due to the slab portion constraining
the modal extent in the vertical direction. Fully suspended
taper geometries may be employed in the future to enhance
the performance at shorter wavelengths.

Waveguide bends can be included in an integrated pho- 3

tonic system, but some material platforms intrinsically affect
bending performance based on tire lateral or horizontal
index contrast, or the typical mode volume required to
realize low-loss waveguides. For waveguides that take
advantage of strong modal confinement to avoid, cladding
losses, relatively large, graded-curvature bends are required
to avoid modal mixing. In other platforms where limited
index contrast is available, bends are also constrained by
leakage losses. With suspended AlGaAs on silicon, however,
a strong index contrast can be achieved on all sides of the
waveguide, enabling small mode volumes and compact
bends. We utilized the structures shown in FIG. 11(d) to test
the bending performance. They consist of a reference path
with only a straight waveguide, and an adjacent path con-
taining 40 or 80 90° bends, each separated by 10 am of
straight waveguide length, to properly assess the mode
transition loss (each bend having two straight-to-bend tran-
sitions). Propagation losses are also part of the total loss.
These structures were repeated over an array of several
widths and radii and tested at both 4=3.4 and 4.6 um. The
bend loss dependence on radius at 4=4.6 jum is shown in
FIG. 11(4), with a typical value of 0.03 dB loss per bend
above radii of 20 jum. The waveguide width was 1.4 um. The
experimental values are plotted alongside simulated bend
loss values, where one bend in simulations consists of two
transitions between straight and bent waveguides. The
results are generally in good agreement, and the discrepancy
in loss values for larger radii can be explained by additional
propagation loss compared to the reference paths. At A=3.4
jum, 0.03 dB loss per bend is experimentally observed for a
radius of 20 um (1.4 um waveguide width), and 0.06 dB loss
per bend at 10 jm radius (1.0 jum waveguide width). For

10

8

2

40

&

18

reference, the simulated loss of one 10 tm radius bend at
4=3.4 wm (two transitions from straight-to-bent or vice
versa) is 0.04 dB, in good agreement, considering the extra
loss is likely from sidewall scattering.

We also investigated 1x2 power splitters, another impor-
tant building block for multifunctional photonic systems in
the mid-IR and LWIR regions. We used MMI-type splitters
for compactness, as opposed to adiabatic or directional
coupler-based designs. The simulated design used the fol-
lowing parameters: MMI length of 6 pan, MMI width of 4
hum, port waveguide width of 1 um, and output port wave-
guide center-to-center gap of 2 jum. The simulated efficiency
was 90% at A=3.4 jun. The optical intensity from a top view
is shown in the lower inset of FIG. 11(e). This device is

5 simulated to maintain better than 90% efficiency (0.46 dB

loss) over an optical bandwidth of 3000-3400 nm. We
characterized the devices with the structures in FIG. 11(e).
They consist of a reference path with only a straight wave-
guide, and an adjacent path containing 12 consecutive 1x2
or 2x1 devices. Comparing their transmission at 4=3.4 um,
we observed an experimental device efficiency of 96#1%.
We also tested several variations in design dimensions, and
the overall performance in FIG. 11(c) was robust to varia-
tions of several hundred nanometers in the MMI length and

s width parameters. The difference in performance between

the simulated and experimental results could be partly a
result of unexpected deviations from the design (such as
rounded features from lithography and etching) and partly
from error in the reference path normalization.

Consider suspended AlGaAs waveguides that are disper-
sion-engineered to achieve anomalous group velocity dis-
persion (GVD) near the pump wavelength of 1560 nm [FIG.
12(c)]. The waveguides have a core width of 480 nm
(estimated based on scanning electron microscopes SEMs
and then increasing the estimated width slightly to obtain
anomalous dispersion in simulations), an effective mode
area of 0.22 um, and a length of 4 mm. Due to a slight
curvature of the etch profile near the waveguide core [FIG.
9(b)]. the mode simulations incorporate a sloped region on
the sidewalls, as shown in the inset of FIG. 12(c). The pump
is an amplified and compressed erbium fiber oscillator with
a repetition rate of 160 MHz, a pulse width of 61 fs, and a
center wavelength of 1560 nm. Light was coupled into the
chip through an aspheric lens, and out through a single-mode

5 lensed fiber. FIG. 12(a) shows the output spectral evolution

as the waveguide-coupled pump pulse energy is varied. At
300 fj pump pulse energy, the waveguide output spectrum
exhibited no noticeable broadening compared to the input
comb spectrum. The onset of supercontinuum generation
was observed near 1.9 pJ, and the spectrum was noticeably
saturated by three-photon absorption (3 PA) at 15 pJ. We
observed octave-spanning supercontinuum generation (at
-45 dB level with respect to pump intensity) at a pulse
energy of 3.4 pJ (average waveguide-coupled power of 0.5

5 mW). We also observed a SHGpeak near 830 nm for almost

all input, power levels. This results from phase-mismatched
conversion over short lengths on the chip and has been
observed in other waveguide platforms possessing nonzero
x. We confirmed this by polarizing the output spectrum in
vertical (TM) or horizontal (TE) directions in FIG. 12(6).
‘The peak was (TM)-polarized. This polarization relationship
(TE pump, TM harmonic) is expected for GaAs or AlGaAs
waveguides propagating along the [OTT] axis. The low pulse
energies required to initiate soliton fission in these wave-

5 guides and the simultaneous production of single-spatial-

mode supercontinuum and SHG could benefit numerous
applications, including frequency combs that could be self-


U.S. Patent Aug. 10, 2021 Sheet 6 of 20 US 11,086,193 B1

200 245
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FIG. 6


U.S. Patent Aug. 10, 2021 Sheet 20 of 20 US 11,086,193 B1

0 ro

&

&

EJ
mJ

‘On-chip conversion efficiency (W"')
g

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Wavelength at SHG peak (am)

Temperature (°C)

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1987 1968 1969 1970 4974
Wavelength (nm)
FIG. 20


US 11,086,193 BI

11

from. Spectrum analyzer 225 can include analog and digital
control and frequency analysis electronic subsystems.

Analyzer signal 226 includes the average frequency of the
beat-note frequency, also referred to as carrier envelope
offset frequency, represented in a digital or analog form for
use by electrical stabilizer 227.

Electrical stabilizer 227 receives analyzer signal 226 and
produces feedback signal 217 therefrom. Electrical stabilizer
227 can include a microprocessor unit, such as an FPGA or
DSP processor, or a computer.

Self-referencing nonlinear frequency converting photonic
waveguide 200 can be made in various ways. In an embodi-
ment a process for making self-referencing nonlinear fre-
quency converting photonic waveguide 200 includes pro-
viding substrate 214; forming dielectric layer 201 on
substrate 214 by thermal oxidation, chemical vapor deposi-
tion, or sputtering; forming a layer of Al,Ga,As. on dielec-
tric layer 201 by epitaxial growth followed by wafer bond-
ing and etch-back of a donor substrate: forming
supercontinuum generator section 202 by: removing a por-
tion of the layer of Al,Ga, As, to form supercontinuum input
optical taper 203 by plasma etching, ion beam milling or wet
chemical etching, removing a second portion of the layer of
Al,Ga,As. to form supercontinuum output optical taper 204
and second harmonic input optical taper 209 by plasma
etching, ion beam milling or wet chemical etching; forming
second harmonic generator section 208 by further removing
material from the layer of Al,Ga,As, to form second har-
monic output optical taper 210 and second harmonic non-

linear optical member 211 by plasma etching, ion beam 3

milling or wet chemical etching; and optionally forming
passivation layer 230 on supercontinuum generator section
202 and second harmonic generator section 208 by sputter-
ing, atomic-layer deposition, chemical vapor deposition, or
epitaxial growth.

It will be appreciated that in making self-referencing

nonlinear frequency converting photonic waveguide 200
elements thereof are arranged with components to be in
communication so that supercontinuum light 207 propagates
from supercontinuum nonlinear optical member 205 with
production of and reception of by and arranging optical
components to be in optical communication and electrical
components to communicate electrical signals amongst the
various electrical components.

Self-referencing nonlinear frequency converting photonic
waveguide 200 has numerous advantageous and unexpected
benefits and uses. In an embodiment, a process for self-
referencing nonlinear frequency conversion with self-refer-
encing nonlinear frequency converting photonic waveguide
200 includes: receiving input light 206 by supercontinuum
input optical taper 203 by coupling to a focused free-space
optical beam or fiber-optic waveguide; communicating input
light 206 from supercontinuum input optical taper 203 to
supercontinuum nonlinear optical member 205 by optical
waveguide propagation; receiving input light 206 by super-
continuum nonlinear optical member 205 by optical wave-
guide propagation: subjecting, by input light 206, input light
206 to anomalous group velocity dispersion by optical
waveguide propagation, producing, by supercontinuum non-
linear optical member 205, supercontinuum light 207 from
input light 206 in response to subjecting input light 206 to
the anomalous group velocity dispersion and birefringent
phase matching in supercontinuum nonlinear optical mem-
ber 205 by optical waveguide propagation, communicating
supercontinuum light 207 and input light 206 from super-
continuum nonlinear optical member 205 to supercontinuum
output optical taper 204 by optical waveguide propagation;

40

0

12

receiving input light 206 and supercontinuum light 207 by
supercontinuum output optical taper 204 and communicat-
ing supercontinuum light 207 and input light 206 from
supercontinuum output optical taper 204 to second harmonic
input optical taper 209 by optical waveguide propagation;
receiving input light 206 and supercontinuum light 207 by
second harmonic input optical taper 209 and communicating
supercontinuum light 207 and input light 206 from second
harmonic input optical taper 209 to second harmonic non-
linear optical member 211 by optical waveguide propaga-
tion; receiving supercontinuum light 207 and input light 206
by second harmonic nonlinear optical member 211 and
producing, by second harmonic nonlinear optical member
211, second harmonic light 212 from supercontinuum light
207 and input light 206 by optical waveguide propagation;
and communicating second harmonic light 212, supercon-
tinuum light 207, and input light 206 from self-referencing
nonlinear frequency converting photonic waveguide 200 as
output light 213 by coupling to a free-space beam or an
fiber-optic waveguide by propagation through the output
optical taper 210, producing feedback signal 217 based on
output light 213 by optical interference and electrical detec-
tion on photoreceiver 223; and producing input light 206
that is feedback stabilized by feedback signal 217

In the process for self-referencing nonlinear frequency
conversion, producing, by the supercontinuum nonlinear
optical member 205, the supercontinuum light 207 includes
making an optical octave in the supercontinuum nonlinear
optical member 205 from the input light 206 by nonlinear
optical broadening processes including four-wave-mixing,
self-phase modulation, and others. The spectrum spans at
least one optical octave, the frequency range including, e.g.,
2.4 times the input light 206 frequency to 0.4 times the pump
frequency, or more specifically 2.4 times the input light 206
to 0.8 times the pump frequency, or alternatively 1.7 times
the pump frequency to 0.6 times the pump frequency, or any
subset of these frequency ranges spanning at least one
octave in frequency.

In the process for self-referencing nonlinear frequency
conversion producing, by second harmonic nonlinear optical
member 211, second harmonic light 212 includes frequency
doubling a low frequency range of supercontinuum light 207
nonlinear interaction with the Al,Ga,As, material via sec-
ond-order optical nonlinearities.

In an embodiment, the process for self-referencing non-
linear frequency conversion further includes measuring the
frequency of the carrier envelop offset by comparing second
harmonic light 212 to a high frequency range of supercon-
tinuum light 207 to perform f-to-2f self-referencing by
optical interference and subsequent electrical detection. As
used herein f-to-2f refers to the optical doubling of an optical
frequency. Further, self-referencing the comparison and
determination of the carrier-envelope offset of a mode-
locked laser system.

In an embodiment, the process for self-referencing non-
linear frequency conversion further includes receiving, by
wavelength splitter 218, output light 213 from self-referenc-
ing nonlinear frequency converting photonic waveguide 200
by optical coupling; and producing, by wavelength splitter
218, combination signal 222 and broadened spectrum 219
from output light 213 by optical spectral demultiplexing.

In an embodiment, the process for self-referencing non-
linear frequency conversion further includes receiving, by
photodetector 223, combination signal 222 from wavelength

5. splitter 218 by optical coupling, and producing, by photo-

detector 223, photosignal 224 from combination signal 222
by optical interference and electrical detection.


U.S. Patent Aug. 10, 2021 Sheet 2 of 20 US 11,086,193 B1

FIG, 2


U.S. Patent Aug. 10, 2021 Sheet 13 of 20 US 11,086,193 B1

Power eH ad

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FIG. 13


U.S. Patent Aug. 10, 2021 Sheet 15 of 20 US 11,086,193 B1

Effective index

SHG efficiency. 9 (W°")

2.20
2.18
2.10

1.90 a
1.85 be
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140 142 144 «4146 «4148 «4150 0« 1520154156
Height (nm)

FIG. 15


U.S. Patent Aug. 10, 2021 Sheet 1 of 20 US 11,086,193 B1

x
Ly
(b) wu 407 21 yt 202 407


US 11,086,193 BI

31
feedback signal to a laser source that produces the input
light based on the feedback signal
11. The detector of claim 10 further comprising the laser
source that produces the input light based on the feedback
signal from the electrical stabilizer before communicating
the input light to the self-referencing nonlinear frequency
converting photonic waveguide.
12. A process for self-referencing nonlinear frequency

conversion with the self-efereneing nonlinear frequency

converting photonic waveguide of claim 1, the process
comprising:
receiving the input light by the supercontinuum input
optical taper;
communicating the input light from the supercontinuum:
input optical taper to the supercontinuum nonlinear
optical member;
receiving the input light by the supercontinuum nonlinear
optical member
subjecting, by the input light, the input light to anomalous
group velocity dispersion;
producing, by the supercontinuum nonlinear optical mem-
ber, the supercontinuum light from the input light in
response to subjecting the input light to the anomalous
group velocity dispersion and birefringent phase
matching in the supercontinuum nonlinear optical
member;
communicating the supercontinuum light and the input

light from the supercontinuum nonlinear optical mem-

ber to the supercontinuum output optical taper;
receiving the input light and the supercontinuum light by

the supercontinuum output optical taper and commu-

nicating the supercontinuum light and the input light

from the supercontinuum output optical taper to the

second harmonic input optical taper;

receiving the input light and the supercontinuum light by
the second harmonic input optical taper and commu-
nicating the supercontinuum light and the input light
from the second harmonic input optical taper to the
second harmonic nonlinear optical member;

receiving the supercontinuum light and the input light by
the second harmonic nonlinear optical member and
producing, by the second harmonic nonlinear optical
member, the second harmonic light from the supercon-
tinvum light and the input light; and

40

32
communicating the second harmonic light, the supercon-
tinuum light, and the input light from the self-referenc-
ing nonlinear frequency converting photonic wave-
guide as output light;

producing a feedback signal based on the output light; and

producing the input light that is feedback stabilized by the

feedback signal prior to receiving the input light by the
supercontinuum input optical taper to perform ‘self-
referencing nonlinear frequency conversion.

13. The process of claim 12, wherein producing, by the
supercontinuum nonlinear optical member, the supercon-
tinuum light comprises making an optical octave in the
supercontinuum nonlinear optical member from the input
light.

14. The process of claim 13, wherein producing, by the
second harmonic nonlinear optical member, the second
harmonic light comprises frequency doubling a low fre-
quency range of the supercontinuum light.

15. The process of claim 14, further comprising measur-
ing the frequency of the carrier envelop offset by comparing
the second harmonic light to a high frequency range of the
supercontinuum light to perform f-to-2f self-referencing.

16. The process of claim 12, farther comprising:

receiving, by the wavelength splitter, the output light from

the self-referencing nonlinear frequency converting
photonic waveguide; and

producing, by the wavelength splitter, the combination

signal and broadened spectrum from the output light.

17. The process of claim 12, further comprising:

receiving, by the photodetector, the combination signal

from the wavelength splitter; and

producing, by the photodetector, the photosignal from the

combination signal.

18. The process of claim 12, farther comprising:

receiving, by the spectrum analyzer, the photosignal; and

producing, by the spectrum analyzer, the analyzer signal
from the photosignal.

19. The process of claim 12, farther comprising:

receiving, by the electrical stabilizer, the analyzer signal

from the spectrum analyzer;

producing, by the electrical stabilizer, a feedback signal

from the analyzer signal;

communicating, by the electrical stabilizer, the feedback

signal to a laser source; and

producing, by the laser source, the input light based on the

feedback signal.

+ ke ee


US 11,086,193 BI

7

toward output terminus 229 opposingly disposed from input
terminus 228; supercontinuum generator section 202 dis-
posed at input terminus 228 on dielectric layer 201 and
including: Al,Ga,As.; supercontinuum input optical taper
203 disposed at input terminus 228 and that receives input
light 206, supercontinuum input optical taper 203 tapering
toward input terminus 228; supercontinuum nonlinear opti-
cal member 205 in optical communication with supercon-
tinuum input optical taper 203 and that receives input light
206 from supercontinuum input optical taper 203 and pro-
duces supercontinuum light 207 from input light 206, super-
continuum light 207 spanning an optical octave with respect
to input light 206; and supercontinuum output optical taper
204 in optical communication with supercontinuum nonlin-
ear optical member 205 and that receives supercontinuum
light 207 and input light 206 from supercontinuum nonlinear
optical member 205 and propagates supercontinuum light
207 and input light 206 from supercontinuum generator
section 202, supercontinuum output optical taper 204 taper-
ing away from input terminus 228, first height H1 relative to
a surface of dielectric layer 201 on which supercontinuum
generator section 202 is disposed, first width W1 of super-
continuum nonlinear optical member 205 orthogonal to a
direction propagation of supercontinuum light 207 in super-
continuum generator section 202, and second harmonic
generator section 208 disposed on supercontinuum output
optical taper 204 and dielectric layer 201 and that extends on
dielectric layer 201 from supercontinuum output optical
taper 204 towards output terminus 229 and including: Al,-
Ga,As_ that provide birefringent modal phase matching;
second harmonic input optical taper 209 disposed on super-
continuum output optical taper 204 and that receives super-
continuum light 207 and input light 206 from supercon-
tinuum output optical taper 204, second harmonic input

optical taper 209 tapering toward supercontinuum output 3

optical taper 204; second harmonic nonlinear optical mem-
ber 211 in optical communication with second harmonic
input optical taper 209 and that receives supercontinuum
light 207 and input light 206 from second harmonic input
optical taper 209 and produces second harmonic light 212
from supercontinuum light 207 and input light 206, second
harmonic light 212 including a second harmonic of super-
continuum light 207; second harmonic output optical taper
210 disposed at output terminus 229 in optical communica-
tion with second harmonic nonlinear optical member 211
and that receives second harmonic light 212, supercon-
tinuum light 207. and input light 206 from second harmonic
nonlinear optical member 211 and co-propagates second
harmonic light 212, supercontinuum light 207, and input
light 206 from second harmonic generator section 208
toward output terminus 229 as output light 213, second
harmonic output optical taper 210 tapering away from
second harmonic nonlinear optical member 211 toward
output terminus 229; second height H2 relative to the
surface of dielectric layer 201 on which second harmonic
generator section 208 is disposed; second width W2 of
supercontinuum nonlinear optical member 205 orthogonal
to a direction propagation of second harmonic light 212 in
second harmonic generator section 208, such that first height
H1 is greater than or equal to second height H2, and first
width W1 is less than or equal to second width W2.
Components of self-referencing nonlinear frequency con-
verting photonic waveguide 200 can be made from and
include various materials. Substrate 214 provides mechani-
cal structure for disposition of other elements of self-
referencing nonlinear frequency converting photonic wave-
guide 200 thereon. Substrate 214 can include a

30

40

&

0

8

semiconductors such as silicon or gallium arsenide, metal,
plastic, glass, ceramic, polymer, and the like. A thickness of
substrate 214 can be from 100 jum to 1 cm, specifically from
10 jum to 2 mm, and more specifically from 300 jum to 1000
jun. In an embodiment, substrate 214 is silicon.

Dielectric layer 201 is disposed on substrate 214 and
optimizes the guided optical mode through supercontinuum
generator section 202 and second harmonic generator sec-
tion 208. Dielectric layer 201 can include a high dielectric
material such as a condensed medium (e.g.., silicon dioxide
as shown in FIG. 1-FIG. 5), gas (e.g... air as shown in FIG.
6), and the like. A thickness of dielectric layer 201 can be
from 500 nm to 100 pm, specifically from 1 pm to 10 pm,
and more specifically from 1 jm to 4 um. In an embodiment,
dielectric layer 201 is silicon dioxide. In certain embodi-
ments, with reference to FIG. 6, dielectric layer 201 is a gas,
and supercontinuum generator section 202 and second har-
monic generator section 208 are suspended from feedback
signal 217 and separated by a gap filled with the gas. A
pressure of the gas can be from le-9 Torr to 5000 Torr,
specifically from 1e-3 Torr to 1000 Torr.

Supercontinuum generator section 202 and second har-
monic generator section 208 are monolithic, made of a
single piece of Al,Ga,As. that provides a nonlinear optical
medium and provides birefringent modal phase matching.
As a result, light (e.g., input light 206, supercontinuum light
207, second harmonic light 212) propagating in supercon-
tinuum generator section 202 or second harmonic generator
section 208 has an optical mode with anomalous group
velocity dispersion provided by Al,Ga,As.. Supercontinuum
input optical taper 203 and second harmonic generator
section 208 independently provide a selected retractive
index with low optical loss so the optical mode is efficiently
guided through respective supercontinuum input optical
taper 203 and second harmonic generator section 208 with-
out radiating light toward dielectric layer 201 or substrate
214. Supercontinuum input optical taper 203 and second
harmonic generator section 208 independently can have an
elemental composition of aluminum in an amount from 0
atomic percent (at. %) to 99.9 at. %, specifically from 1 at.
% to 50 at. %, and more specifically from 5 at. % to 40 at.
%, based on a total of all atoms in this layer; gallium in an
amount from | at. % to 55 at. %, specifically from 10 at. %
to 55 at. %, based on a total of all atoms in this layer, and
arsenic in an amount from 1 at. % to 55 at. %, specifically
from 10 at. % to 55 at. %, based on a total of all atoms in
this layer. First height H1 of supercontinuum input optical
taper 203 can be from 50 nm to 5000 nm, specifically from
80 nm to 1000 nm, and more specifically from 100 nm to
400 nm. First width W1 of supercontinuum input optical
taper 203 can be from 100 nm to 5000 nm, specifically from
200 nm to 2000 nm, and more specifically from 250 nm to
800 nm. Second height H2 of supercontinuum output optical
taper 204 can be from 50 nm to 2000 nm, specifically from

5 80 nm to 500 nm, and more specifically from 90 nm to 400

nm. Second width W2 of supercontinuum output optical
taper 204 can be from 200 nm to 6000 nm, specifically from
300 nm to 2000 nm, and more specifically from 1000 nm to
1600 nm.

It is contemplated that propagation length in the direction
of the Y-axis in FI. 2 of supercontinuum generator section
202 and second harmonic generator section 208 indepen-
dently can be from 10 um to 200 mm, specifically from 100
uum to 20 mm, and more specifically from 1 mm to 10 mm.

Supercontinuum generator section 202 includes supercon-
tinuum input optical taper 203 and supercontinuum output
optical taper 204 between which is interposed supercon-


US 11,086,193 B1
Page 2

(56) References Cited

OTHER PUBLICATIONS

M. Malinowski et al., “Amplified octave-spanning supercontinuum
from chalcogenide waveguides for second-harmonic generation,”
2017 IEEE Photonics Conference (IPC), Orlando, FL, 2017, pp.
261-262. (Year: 2017).*

Hickstein, D., et al., “Ultrabroadband Supercontinuum Generation
and Frequency-Comb Stabilization Using On-Chip Waveguides
with Both Cubic and Quadratic Nonlinearities”, Physical Review
Applied, 2017, p. 014025, vol. 8.

Yu, M., et al., “Coherent two-octave-spanning supercontinuum
generation in lithium-niobate waveguides”, Optics Letters, 2019, p.
1222-1225, vol. 44 No. 5.

Jankowski, M,, et al., “Ultrabroadband nonlinear optics in nanophotonic
periodically poled lithium niobate waveguides”, Optica, 2020, p.
40-46, vol. 7 No. 1.

Okawachi, Y., et al., “Chip-based self-referencing using integrated
lithium niobate waveguides”, Optica. 2020, p. 702-707, vol. 7 No.
6

Stanton, E., etal.,"Eflicient second harmonic generation in nanophotonic
GaAs-on-insulator waveguides”, Optics Express, 2020, p. 9521-
9532, vol. 28 No. 7

Chiles, J. et al., “Multifunctional integrated photonics in the
mid-infrared with suspended AlGaAs on silicon”, Optica, 2019, p.
1246-1252, vol. 6 No. 9

* cited by examiner


US 11,086,193 BI

23

wherein dx is the spatially varying effective second-order
nonlinear coefficient corresponding to components of the rid
tensor that interact with xx-polarized pump light. The xx-
polarized component of the pump mode is Ex,, and the
y-polarized component of the signal mode is Ey.2o. Wave
propagation is in the z-direction and the waveguide top-
surface in the y-direction. Each integral in Eq. (3) is taken
over all space in the x-y plane.

‘The magnitude of the d,, and d,, nonlinear coefficients of
GaAs are equivalent for a (100) oriented top surface with
‘wave propagation in the plane of the (OTT) primary flat or the
(0T1) secondary flat. However, there is a wide range of
reported values for this coefficient, and these values have
large uncertainties because of the measurement techniques
and differences in material preparations. For the calculation
in this work, we use d.=180 pm/V for GaAs to approxi-
mately match our measurements, and the value is neglected
in SiO, and air. Guided optical modes are calculated using
a finite-difference method eigenmode solver from which x is
evaluated by Eq. (3). The GaAs is partially etched so a 15
nm thick slab remains, and it is cladded with SiO, on bottom.
and air on top. The effective refractive indices, n,, and no...
and the nonlinear coupling, K, are calculated for different
heights and widths of the waveguide core. A Sellmeier
model is used for the index of each material. The phase-
matched geometries are found from the intersection of the
discretely calculated values for n,, of the transverse-electric
(TE) polarized pump and n,,, of the transverse-magnetic
(TM) polarized signal. Similarly, x is found by interpolation.

FIG. 15(a) shows the effective indices of the pump and 3

signal light for a range of heights and widths, and FIG. 15(h)
shows the conversion efficiency 7 for a length of 2.9 mm
(the length used in this experiment) with the corresponding
phase-matched geometries. The peak simulated conversion

efficiency is 50.3 W~* which occurs at a GaAs thickness of 3

149 nm and a width of 1.3. The pump and signal propagation
losses used in this simulation are 1.5 dB/cm and 16.8 dB/em,
respectively.

Phase error between the pump and signal light due to
width and height variations of the waveguide degrade the
signal and typically limit the conversion efficiency of wave-
guided SHG devices. This effect is analyzed using the
effective indices in FIG. 15(a). The FWHM bandwidths of
the SHG signal are calculated for variations with waveguide
width (Aw) and height (Ah), which are plotted in FIG. 16 for
a length of 2.9 mm. For larger heights, Aw increases,
indicating greater tolerance to width variations. However,
the trend for Ah is relatively constant for this range of
waveguide heights. Although the peak conversion efficiency
may be lower, an SHG waveguide with a height greater than
149 nm is less sensitive to geometric variations. For longer
waveguides, these phase errors accumulate and the conver-
sion efficiency becomes more sensitive to geometric varia-
tions. Our design targets a thickness of 154 nm for the SHG
experiment, which has a phase-matched width of 1.9 and a
maximum possible conversion efficiency 40.5 W~! as shown
in FIG. 15(4).

The input, facet waveguide width is 490 nm to minimize
the coupling loss of the pump mode from a tensed fiber,
which is simulated to be 3.6 dB. This waveguide tapers to
the SHG waveguide width over a length of 100. At the
output facet the waveguide width is the same as the SHG
section. The wide waveguide at the output reduces the facet
coupling uncertainty compared to a tapered waveguide and
the input facet taper is sufficient to suppress unwanted
internal reflections. The simulated efficiency for the output
coupling of the signal mode is 7.7 dB, however, even higher

0

40

0

24

loss is expected because this calculation does not account for
higher-order mode coupling in the output fiber.

While GaAs and AlGaAs are ideal materials for SHG
because of their large second-order nonlinearity, two factors
limit their performance and application space. First, the
bandgap of GaAs limits the SHG signal wavelength to >900
nm. For AlGaAs, shorter wavelengths down to ~600 nm can
be achieved depending on the Al mole fraction. The second
limiting factor is the surface defect-state absorption. Spe-
cifically, As—As bonds formed at the surface create a strong
absorption feature with a peak near 950 nm. For GaAs, this
absorption feature limits the minimum signal wavelength to
~950 nm. Because of the larger refractive index, GaAs
supports higher confinement and smaller modes compared to
AlGaAs compounds. Therefore, we use GaAs since its
bandgap is compatible with our application to convert ~2.0
light to ~1.0.

FIG. 17(a) shows a schematic of the designed waveguide
cross-section. The fabrication process begins with molecular
beam epitaxial growth on the (100) surface of a GaAs wafer.
A 150 nm thick Al gGay »As etch-stop layer is grown first,
followed by the 158 nm thick GaAs waveguide layer. The
waveguide layer is 4 nm thicker than is intended for the
fabricated waveguide to account for material loss in subse-
quent HCI and NH,OH steps. This GaAs wafer is then
bonded to a Si wafer with a 3.0 thick thermal oxide layer.
Before bonding, both wafers are cleaned with solvents and
activated with an atmospheric plasma containing metastable
He and free-radicals of H and N. The bond is initiated in air
at room temperature using a custom-built wafer bonding
apparatus, and an anneal at 150° C. ona hotplate for 1 hour
completes the bond. Separate bonding experiments are used
to measure the bond energy of (0.91+0.16) J/m? with the
double cantilever beam method. While this is leaver than
other bonding recipes using SiN or Al,O, interlayers, this
direct bond minimizes the waveguide loss and is strong
enough to complete the waveguide fabrication.

The GaAs substrate is removed in two steps. First, the
bulk of the substrate is etched with a mixture of 350 mL
HO, (9.8 mol/L) and 25 mL NH,OH (14.5 mol/L) using an
N, bubbler to stabilize the reaction. After the 150 nm thick
Alp goGo.2oAs etch-stop layer becomes visible (typically
after 80 minutes), the wafer is transferred to a second
mixture (also using an N, bubbler) with the same H,O,, but
only 10 mL NH,OH. Residue is removed with HCI (2.0
mol/L) for 20 s, and the etch-stop is selectively etched with
HP (1.4 mol/L) for 30 s. Residual fluoride compounds
produced from the HF etch are removed with A-methyl-2-
pyrrolidone at 80° C. for 5 min and then NH,OH (1.3 mol/L)
for 1 min. An image of the Si wafer at this point in the
process with the transferred GaAs film is shown in FIG.
17(b). The RMS surface roughness, measured with atomic
force microscopy, on top of the GaAs is ~0.2. nm over 9 um?,
which is the same as the bottom surface before bonding. The

5. thickness uniformity is measured with optical interferometry

to be +0.2% or +0.3 nm within a 20 mm radius from the
wafer center.

The GaAs waveguides are defined with electron-beam
lithography and inductively-coupled, plasma (TCP) reac-
tive-ion etching (RIE). The etch uses BCI, with 2.00 W TCP
power, 100 W bias power, 5 mTorr chamber pressure, and a
temperature of 20° C. By dividing the etch in two steps and
measuring the GaAs thickness in between, we consistently
leave a 15 nm thick GaAs slab with an error of +1 nm,

5 limited by the optical interferometric measurement. This

GaAs slab is important to prevent exposure of the GaAs/
SiO, interface to HCl, which etches the interface non-


US 11,086,193 BI

13

In an embodiment, the process for self-referencing non-
linear frequency conversion further includes receiving, by
spectrum analyzer 225, photosignal 224 by electrical cou-
pling; and producing, by spectrum analyzer 225, analyzer
signal 226 from photosignal 224 by electrical coupling.

In an embodiment, the process for self-referencing non-
linear frequency conversion further includes receiving, by
electrical stabilizer 227, analyzer signal 226 from spectrum
analyzer 225 by electrical coupling; producing, by electrical
stabilizer 227, feedback signal 217 from analyzer signal 226
by digital or analog signal processing or transduction to
digital electronic or analog electronic or the optical domain;
communicating, by electrical stabilizer 227, feedback signal
217 to laser source 216 by electrical or optical coupling; and
producing, by laser source 216, input light 206 based on
feedback signal 217 by adjustment of the effective cavity
length in the laser source 216

Self-referencing nonlinear frequency converting photonic
waveguide 200 and processes disclosed herein have numer-
ous beneficial uses, including realizing a coherent and
stabilized broadband optical frequency spectrum. Advanta-
geously, self-referencing nonlinear frequency converting
photonic waveguide 200 overcomes limitations of technical
deficiencies of conventional compositions such as fabrica-
tion complexity (by a single layer of Al,Ga,As, which
provides the optical nonlinearity) and efficient coupling
between all elements of self-referencing nonlinear fre-
quency converting photonic waveguide 200.

It should be appreciated that self-referencing nonlinear

frequency converting photonic waveguide 200 and self-

referencing nonlinear frequency conversion solves the prob-
lem of inherently high power consumption and physical
space consumed by conventional f-to-2f self-referencing
devices and methods. Conventional devices include ppLN

crystals that suffer significant coupling losses and operate 3

under high threshold optical power, wherein nonlinear
broadening can require an optical amplifier to reach a level
of optical intensity that can be required to broaden light to
‘one octave in conventional nonlinear fiber. Advantageously,
self-referencing nonlinear frequency converting photonic
waveguide 200 includes Al,Ga,As, waveguides to produce
octave-spanning supercontinuum spectra and simultane-
ously achieve second-harmonic generation at pump pulse
energies compatible with a fiber mode-locked oscillator
operating at a repetition rate of ~100-200 MHz in an absence
of optical amplification and provides much smaller devices
with very low power consumption compared to conventional
designs. In an embodiment, octave-spanning supercon-
tinuum generation and simultaneous second-harmonic gen-
eration occurs at pump pulse energies of 3 pJ or 0.5 mW of
average power with Al,Ga,As. nanophotonic waveguides
that have thickness uniformity for efficient and predictable
second harmonic generation, high second-order optical non-
linearity compared with many crystalline materials, high
third-order nonlinearity for efficient supercontinuum gen-
eration, compact mode volume for reduction in minimum
pump pulse energy with fewer components for f-to-2f self-
referencing with comparison to conventional devices in an
absence of the Al,Ga,As, waveguides

Self-referencing nonlinear frequency converting photonic
waveguide 200 and processes herein unexpectedly provides
components for realization of a self-reference mode-locked
laser system or frequency comb with low power consump-
tion, by taking advantage of the strong third- and second-
order optical nonlinearities which can be simultaneously
designed to be taken advantage of in a sequence on one
monolithic optical chip. By utilizing Al,Ga,As. as the core

S

3

40

&

14

material of the waveguide, the strong waveguide dispersion
can be controlled due to refractive index contrast of the
surrounding optical medium to provide engineered super-
continuum production. Moreover, due to the nonlinear opti-
cal tensor of Al,Ga,As., the waveguide cross-section can be
engineered to produce birefringent modal phase matching of
a fundamental and second harmonic optical mode in
orthogonal polarization modes, providing simple realization
of modal phase matching that can be difficult to produce in
conventional optical media, as conventionally done with
more complex processes such as higher-order optical modal
phase matching, periodic poling, and the like.

The articles and processes herein are illustrated further by
the following Examples, which are non-limiting,

EXAMPLES
Example 1

Microscale integration of mid- and long-wave-infrared
photonics could enable the development of biddable, robust
chemical sensors, as well as highly efficient infrared fre-
quency converters. However, such technology would be
defined by the choice of material platform, which immedi-

5 ately determines the strength and types of optical nonlin-

earities available, the optical transparency window, modal
confinement, and physical robustness. Suspended Al,Ga,As.
waveguides integrated on silicon provide excellent perfor-
mance in all of these metrics. We demonstrate low propa-
gation losses within a span of nearly two octaves (1.26-4.6
uum) with exemplary performance of 0.45 dB/cm at 4=2.4
yum. We exploit the high nonlinearity of this platform to
demonstrate 1560 nm-pumped second-harmonic generation
and octave-spanning supercontinuum reaching out to 2.3 um
with 3.4 pJ pump pulse energy. With mid-IR pumping, we
generate supercontinuum spanning from 2.3 to 6.5 pm.
Finally, we demonstrate the versatility of the platform with
mid-infrared passive devices such as low-loss 10 um-radius
bends, compact power splitters with 96+1% efficiency, and
edge couplers with 3.01 dB loss. This platform has strong
potential for multifunctional integrated photonic systems in
the mid-infrared.

The mid- and long-wave-infrared (mid-IR and LWTR)
spectral regions from A=3-8 and 8-15 uum are critical spectral
regions for sensitive spectroscopic analysis of a variety of
physical compounds, such as complex molecular solids,
gaseous species, and liquid mixtures. Optical frequency
combs in particular have been used to achieve broadband
spectroscopy with exquisite frequency resolution. However,
broadband mid-IR comb technology is still maturing, and
nearly all systems would greatly benefit from increased
spectral coverage, lower power operation, and improved
robustness. Consequently, there has been a consistent push
over the past decade to transition mid-IR comb systems or
subsystems to compact and robust chip-scale platforms. In
particular, the small modal area and long propagation
lengths of integrated nanophotonic waveguides motivate the
development of integrated frequency converters to extend
the spectral reach of combs, often with negligible power
burdens.

Development of integrated mid-IR photonic systems for
these applications depends on many factors, such as the
material platform. A subset of materials has suitable optical
transparency, and strong optical nonlinearities are also
required for the generation, or broadening of frequency
combs in the mid-IR. While Kerr nonlinearity is present in
silicon, germanium, and chalcogenide materials, they lack



U.S. Patent Aug. 10, 2021 Sheet 11 of 20 US 11,086,193 B1

% Measures

WR tengih = SS gon |
@ PAS eegth # 6.0 nee!

rr. rrr

FIG. 11


U.S. Patent Aug. 10, 2021 Sheet 14 of 20 US 11,086,193 B1

This work: GaAs-O}

3 0 . ALN RPE

Conversion efficiency (W7})

w= 40°40" 1° = 10” 40°
Bandwidth of signal, BW (GHz)

FIG. 14