78
| Nature | Vol 620 | 10 August 2023
Article
3D integration enables ultralow-noise
isolator-free lasers in silicon photonics
Chao Xiang
1,2
,7
✉
, Warren
Jin
1,3
,7
, Osama
Terra
1,6
,7
, Bozhang
Dong
1,7
, Heming
Wang
1
, Lue
Wu
4
,
Joel Guo
1
, Theodore J. Morin
1
, Eamonn
Hughes
5
, Jonathan
Peters
1
, Qing-Xin
Ji
4
, Avi
Feshali
3
,
Mario Paniccia
3
, Kerry J. Vahala
4
& John
E.
Bowers
1,5
✉
Photonic integrated circuits are widely used in applications such as
telecommunications and data-centre interconnects
1
–
5
. However, in optical systems
such as microwave synthesizers
6
, optical gyroscopes
7
and atomic clocks
8
, photonic
integrated circuits are still considered inferior solutions despite their advantages in
size, weight, power consumption and cost. Such high-precision and highly coherent
applications favour ultralow-noise laser sources to be integrated with other photonic
components in a compact and robustly aligned format—that is, on a single chip—for
photonic integrated circuits to replace bulk optics and fibres. There are two major
issues preventing the realization of such envisioned photonic integrated circuits: the
high phase noise of semiconductor lasers and the difficulty of integrating optical
isolators directly on-chip. Here we challenge this convention by leveraging three-
dimensional integration that results in ultralow-noise lasers with isolator-free
operation for silicon photonics. Through multiple monolithic and heterogeneous
processing sequences, direct on-chip integration of III–V gain medium and
ultralow-loss silicon nitride waveguides with optical loss around 0.5 decibels per
metre are demonstrated. Consequently, the demonstrated photonic integrated
circuit enters a regime that gives rise to ultralow-noise lasers and microwave
synthesizers without the need for optical isolators, owing to the ultrahigh-quality-
factor cavity. Such photonic integrated circuits also offer superior scalability for
complex functionalities and volume production, as well as improved stability and
reliability over time. The three-dimensional integration on ultralow-loss photonic
integrated circuits thus marks a critical step towards complex systems and networks
on silicon.
Following the path of electronic integrated circuits (EICs), silicon (Si)
photonics holds promises to enable photonic integrated circuits (PICs)
with high densities, advanced functionality and portability. Although
various Si photonics foundries are rapidly developing PIC capabilities—
enabling volume production of modulators, photodetectors and
most recently lasers—Si PICs have yet to achieve the stringent require
-
ments on laser noise and overall system stability imposed by many
applications such as microwave oscillators, atomic physics and pre
-
cision metrology
9
–
11
. Semiconductor lasers must strongly suppress
amplified-spontaneous-emission noise to achieve narrow linewidth
for these applications
12
. They will also require isolation from the rest
of the optical system, otherwise the laser source will be sensitive to
back-reflections from downstream optical components that are beyond
the control of the PIC designer
13
. In many integrated photonic solutions,
a bulk optical isolator must be inserted between the laser chip and the
rest of the system, significantly increasing the complexity, as well as
the cost of assembly and packaging
14
.
To enrich the capabilities of Si PICs and avoid multi-chip optical
packaging, non-group-IV materials need to be heterogeneously inte
-
grated to enable crucial devices, including high-performance lasers,
amplifiers and isolators
15
–
17
. It has now been widely acknowledged that
group III–V materials are required to provide efficient optical gain
for semiconductor lasers and amplifiers in Si photonics regardless
of the integration architecture, but concerns still remain for a com
-
plementary metal–oxide–semiconductor (CMOS) fab to incorporate
magnetic materials, which are currently used in industry-standard
optical isolators
18
.
Fortunately, a synergistic path towards ultralow laser noise and low
feedback sensitivity exists—using ultrahigh-quality-factor (
Q
) cavities
for lasers that not only reduce the phase noise but also enhance the feed
-
back tolerance to downstream links. These effects scale with the cavity
Q
and ultrahigh-
Q
cavities would thus endow integrated lasers with
unprecedented coherence and stability
19
,
20
. The significance is twofold.
First, the direct integration of ultralow-noise lasers on Si PICs without
https://doi.org/10.1038/s41586-023-06251-w
Received: 26 December 2022
Accepted: 23 May 2023
Published online: 2 August 2023
Open access
Check for updates
1
Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, CA, USA.
2
Department of Electrical and Electronic Engineering, The University of
Hong Kong, Hong Kong, China.
3
Anello Photonics, Santa Clara, CA, USA.
4
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA, USA.
5
Materials
Department, University of California, Santa Barbara, Santa Barbara, CA, USA.
6
Present address: Primary Length and Laser Technology Lab, National Institute of Standards, Giza, Egypt.
7
These
authors contributed equally: Chao Xiang, Warren Jin, Osama Terra, Bozhang Dong.
✉
e-mail:
cxiang@eee.hku.hk
;
bowers@ece.ucsb.edu
Nature | Vol 620 | 10 August 2023 |
79
the need for optical isolators simplifies PIC fabrication and packaging.
Furthermore, this approach does not introduce magnetic materials
to a CMOS fab as isolators are not obligatory for such complete PICs.
3D integration of lasers and ultralow-loss PICs
Now we consider developing an integration architecture and process
flow to seamlessly integrate the III–V-based lasers with ultralow-loss
(ULL) waveguides. Among various ULL integrated photonics platforms,
silicon nitride (SiN) has emerged as the leading performer and ena
-
bled a series of breakthroughs in metrology, sensing and telecom
-
munications
21
–
23
. To achieve ultralow waveguide loss, SiN waveguides
require high-temperature annealing
24
–
27
that violates the thermal
budget of back-end-of-line semiconductor manufacturing processes.
Front-end-of-line-fabricated ULL SiN waveguides are nonetheless sus-
ceptible to subsequent processing steps that could introduce addi
-
tional loss, especially during heterogeneous laser integration, which
involves multiple etches and depositions. To address these issues,
we propose to use three-dimensional (3D) structures for the integra
-
tion of lasers with ULL waveguides. Recent years have witnessed the
development of 3D integration in electronics by heterogeneously or
monolithically integrating layers for increased circuit densities and
functionalities
28
,
29
. In photonics, 3D integration has been investigated
for monolithic devices (for example, waveguides, modulators and pho
-
todetectors (PDs))
30
and heterogeneously integrated lasers
31
. Here we
combine monolithic and heterogeneous 3D integration to fully unlock
the potential of enabling complex and high-performance photonic
devices and integrated circuits.
We effectively separate a 3D Si PIC into layers with respective pho
-
tonic functionalities, as shown in Fig.
1a
. The designed device consists
of four major functionality layers, including a III–V gain layer, a Si PIC
layer, a SiN redistribution layer (RDL) and a SiN ULL layer. The sepa
-
ration of the Si and the ULL SiN layers is approximately 4.8 μm, such
that the ULL SiN layer can be effectively isolated from subsequent Si
and indium phosphide (InP) processing steps, thereby retaining the
performance of the ULL SiN (Extended Data Figs. 1–3). Such design
necessitates interlayer transition across multiple functional layers.
Unlike EICs that rely on the interlayer metallic vias for interconnects,
3D PICs leverage evanescent coupling across multiple layers and use
waveguide geometry designs to achieve interlayer transitions that are
otherwise forbidden. More specifically, we introduce a photonic RDL
between the Si and ULL SiN layers for the control of coupling between
the top active layers and the bottom ULL passive layer. A highly effi
-
cient, active–passive layer transition can be provided by the RDL where
necessary (Extended Data Figs. 1 and 2).
The cross-section of the 3D PICs is also illustrated in Fig.
1a
, show
-
ing its compatibility with foundry-available Si photonic components,
including Si modulators and Si/germanium (Ge) PDs. In addition,
such PICs could be further heterogeneously integrated with EICs for
high-density 3D E-PICs. In our 3D photonic integration structure, the
thick oxide separation forms an effective barrier for back-end loss
origins such that ultrahigh-
Q
resonators (with intrinsic
Q
≈ 50 million
at the laser wavelength) are fully integrated with high-performance
III–V/Si distributed-feedback (DFB) lasers (Fig.
1b
). It must be noted
that the 3D integration can result in multiple overlapping but decou-
pled photonic functionality layers—a goal not possible in previous
demonstrations of heterogeneous integration
31
,
32
. This decoupling is
now enabled by the large vertical mode separation, which is bridged
by the SiN RDL. The multilayer structure of the fabricated device and
InP epi wafer stack are shown in Fig.
1c
.
Single-chip self-injection locked lasers
We leverage self-injection locking of InP/Si DFB lasers to thermally
tunable SiN ultrahigh-
Q
resonators for ultralow-noise lasers on the
3D Si PIC. The working principle of such a device is summarized in
Fig.
2a
, which requires the laser and ring resonance wavelengths to
match in the frequency domain, as well as the forwards and backwards
signals to phase match in the time domain. To set the device to the
proper working conditions, the InP/Si laser wavelength is tuned by the
applied gain current, the SiN ring resonance is tuned by the thermal
heater, and the forwards and backwards phases are tuned by the ther-
mal phase tuner placed on the Si waveguides. Once both wavelength
and phase-matching conditions are achieved, the free-running laser
locks to the ultrahigh-
Q
resonator owing to Rayleigh backscattering,
resulting in several resonator-defined laser properties (Extended Data
Figs. 4 and 5).
We investigate the dynamics and performance of the self-injection
locked (SIL) laser using the measurement set-up shown in Fig.
2b
.
Owing to the availability of an on-chip phase tuner between the laser
and ring resonator, we can clearly unveil the phase-dependent lock
-
ing dynamics. In previous butt-coupled SIL experiments, tuning the
chip-to-chip phase also varies the coupling loss, that is, the output
power. Because the InP/Si laser and SiN resonator are heterogeneously
integrated together, and the phase is thermally tuned on the chip, these
are now decoupled in our experiment. Figure
2c
shows the dependence
of laser coherence on the phase-tuner power causing the phase shift.
The laser wavelength is preset to match one of the ring resonances. We
can observe a periodic dependence of the laser coherence when the
laser-to-resonator phase is tuned by several one-direction π periods.
Within each period, the laser goes through low-phase-noise locking,
chaotic coherence collapse and high-phase-noise free-running regimes.
These regimes are also observed from the time-domain power trace
recorded on an oscilloscope when the current on the phase tuner is
swept across a full period (Fig.
2c
, bottom).
The ring resonance is another degree of freedom to control the lock
-
ing dynamics. By tuning the thermal tuner current on the ULL SiN ring
in both directions, the laser can be switched between the free-running
state and the locked state as plotted in Fig.
2d
(top). Depending on the
phase, the locking range can be different for the bidirectional sweep.
We observed about 1.4-GHz and 2.4-GHz locking ranges for the bidi
-
rectional sweep. This measured locking range is also affected by the
thermal crosstalk during the resonance tuning, as evidenced by the
laser frequency shift at free-running state. Figure
2d
(bottom) shows
the modelled asymmetric locking range without the thermal crosstalk
at phase-matched conditions. Details of the calculation can be found
in Supplementary Section V.
Ultralow laser frequency noise enabled by self-injection locking
has been extensively studied in recent years
33
. These demonstrations,
however, mostly rely on individual ultrahigh-
Q
resonators, includ
-
ing crystalline whispering-gallery-mode resonators
34
and SiN ring
35
or spiral resonators
36
. The laser and the resonator are thus separate
and need free space or fibre coupling. We recently demonstrated the
self-injection locking of lasers with dispersion-engineered resona
-
tors on a heterogeneous chip for soliton microcomb generation
32
.
However, the laser frequency noise is still relatively high, especially
in the range of 1 kHz to 100 kHz, which is critical to microwave and
sensing applications
37
. Our current device, with around 0.5 dB m
−1
ULL integrated with lasers on the same chip, showed the lowest laser
frequency noise for a single-chip device, with around 250 Hz
2
Hz
−1
and 2.3 Hz
2
Hz
−1
at 10-kHz offset and at the white noise floor, respec
-
tively, for the through port. The white noise floor for the drop port is
even lower (1.7 Hz
2
Hz
−1
), showing about 5-Hz fundamental linewidth.
It needs to be noted that these results are achieved with a relatively
compact 30-GHz free spectral range (FSR) resonator, and the laser
frequency noise is limited by the thermorefractive noise. Using a larger
ring radius or a spiral-shaped resonator to reduce the thermorefrac
-
tive noise, lower frequency noise (for example, subhertz fundamental
laser linewidth) should be achieved using the same design strategy and
fabrication process.
80
| Nature | Vol 620 | 10 August 2023
Article
Cavity-mediated feedback sensitivity
In addition to frequency noise, integration with the ultrahigh-
Q
cavity
markedly reduces the feedback sensitivity
38
. This goal has been pursued
by many demonstrations, but owing to the difficulty of integrating
ultrahigh-
Q
cavities with lasers, the feedback tolerance is limited such
that an isolator is still required to operate in the strong feedback regime
(more than −10 dB)
39
,
40
.
In the current SIL configuration with an add-drop ring resonator, the
laser output can be taken from both the through and the drop ports
(Fig.
3a
). The ring resonator itself acts as an intensity filter for both
forwards output and backwards reflections. This results in another
degree of freedom in controlling the feedback sensitivity by modifying
the loading factor of the ring resonator. The dependence on the feed-
back is characterized using the experimental set-up shown in Fig.
3b
.
The downstream feedback results in the change of laser coherence
for a feedback-sensitive laser. The laser can operate in several differ
-
ent regimes depending on the feedback strength
13
. Stable operation
requires the laser to stay in regime I where the laser coherence is main-
tained. With an increased feedback level, the laser transitions to regime
II, where the linewidth is governed by the feedback phase (the length
of the external cavity). The critical feedback level at the boundary of
regimes I and II (
f
r1
) represents the highest feedback level a laser can
tolerate to maintain stable operation. After the laser enters regime
IV, the laser coherence collapses. Our laser did not enter regime III, in
which a significant frequency stabilization owing to external optical
1,460
1,530
1,565
1,625
Wavelength (nm)
0.1
0.2
0.4
0.6
1.0
Loss (dB m
–1
)
–60
–40
–20
0
Spectral intensity (dBm)
III–V gain
SiN RDL
SiN ULL
Si PIC
SiN ULL
SiO
2
SiO
2
Si
p-InP
p-InGaAs
Pd/Ti/Pd/Au/Ti/Au
Ti/Au
S band
C band
L band
SiN waveguide loss
InP/Si laser spectrum
3D photonic integration
F
oun
d
ry ava
il
a
ble
Heterogeneous
SiN RDL
Si
SiN ULL
S
i modulato
r
S
i
/G
e PD
Si substrate
3D
e
l
ec
tr
o
ni
c-
p
h
oton
i
c
inte
g
ratio
n
Si phase
modulator
Si DFB
3D transitions
1
μ
m
a
b
c
n-InP
In
0.53
Al
0.183
Ga
0.287
As
In
0.85
Ga
0.15
As
0.327
P
0.673
In
0.53
Al
0.183
Ga
0.287
As
p-InP
In
0.6758
Al
0.06
Ga
0.2642
As
In
0.4411
Al
0.085
Ga
0.4739
As
10 nm
III–V gain
DFB lasers
3D transitions
Ultrahigh-
Q
resonators
1 mm
Monolithic
n
Fig. 1 | 3D integrated Si PIC chip.
a
, Left: concept of 3D photonic integration
of functional layers (top) and the corresponding devices on a fabricated 3D
PIC (device picture shown in the bottom). This chip is singulated from a fully
fabricated 100-mm-diameter wafer. The SiN wafer process is performed on a
200-mm-diameter wafer fabricated in a CMOS foundry, which was later cored
into 100-mm-diameter wafers for heterogeneous laser fabrication. Right: the
cross-section of the demonstrated 3D PIC in solid colours. We envision that
future works will enable additional functionality, such as integration with
foundry-available Si modulators and Ge/Si PDs, and 3D electronic–photonic
heterogeneous integration, which are shown in transparent colours. Both
monolithic and heterogeneous integration processes are employed, in which
3D transitions are critical to the vertical integration of functionality layers.
b
, Measured III–V/Si DFB laser spectrum centred at the telecom C band on the
3D PIC (right axis) and measured ULL SiN waveguide loss (left axis, extracted
from the fitted resonator
Q
) across the telecom S, C and L bands on the same
3D PIC.
c
, Left: false-coloured focused ion beam scanning electron microscopy
image of the fabricated 3D PIC showing the laser cross-section. Right:
transmission electron microscopy image showing the layered InP epitaxial
stack after bonding and substrate removal.
Nature | Vol 620 | 10 August 2023 |
81
feedback can take place, regardless of the feedback phase. In general,
regime III is too narrow to be observed in most semiconductor lasers.
We calculated the critical feedback level as a function of the
cavity-loaded
Q
(Fig.
3c
). Subject to different Rayleigh backscatter-
ing strengths (
R
), the laser undergoes variable tolerance to down
-
stream reflection. In general, large high-
Q
feedback (the Rayleigh
scattering from the ultrahigh-
Q
resonator) is beneficial in leading
to high downstream reflection tolerance. This effect saturates at
certain loaded
Q
when the phase response provided by the reso
-
nator cannot compensate for larger reflected powers outside the
resonator.
To experimentally verify the high feedback tolerance owing to the
integrated laser and ultrahigh-
Q
resonator, we studied the laser coher
-
ence dynamics with varied downstream reflection strength, and the
ab
d
e
c
05
10
15
20
25
30
Phase tuner power (mW)
26.8
26.9
27.0
27.1
27.2
Frequency (MHz)
Locked
05
10
15
20
25
Time (s)
0
1
2
3
4
5
Power (a.u.)
Phase current sweep
Free running
Chaotic
PD
PC
EDF
A
EDF
A
ESA
PD
OSC
ISO
Fibre laser
AOM
I
II
III
PD
ESA
OSA
PNA
Laser characterization
SIL characterization
Ring
50
50
50
50
50
50
Ring
Phase
Phase
Laser
Laser
Frequency domain
Laser
Ring
Time domain
t
O
O
Forwards
Backwards
Heater tuning
Laser tuning
–400
–200
0
200
Laser detuning (MHz)
–400
–200
0
200
Effective detuning (MHz)
–500
0
500
1,000
1,500
Laser detuning (MHz)
–500
0
500
1,000
1,500
Effective detuning (MHz)
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Offset frequency (Hz)
10
8
10
6
10
4
10
2
10
0
Frequency noise (Hz
2
Hz
–1
)
Through port
Drop port
30-GHz-ring TR
N
β
separation line
Ref.
32
Noise oor
01
02
03
04
05
06
07
08
0
Time (s)
18
20
22
Beat frequency (GHz)
Locked
Free running
Fig. 2 | Laser self-injection locking and phase noise.
a
, Schematic illustration
of the laser self-injection locking, which requires tuning in wavelength and phase
to work. There are three knobs used to control the working regimes respectively:
laser current, phase heater current and ring heater current.
b
, The experimental
set-up to characterize the laser performance and the self-injection locking
process.
c
, The dependence of laser self-injection locking on the phase-tuner
power. Top: the change in delayed self-heterodyne beat spectrum recorded by
an ESA. Bottom: the corresponding power recorded on the oscilloscope of one
phase tuning period revealing the locked, chaotic and unlocked states. The
acoustic-optic modulator (AOM) used in this experiment has a centre frequency
of 27 MHz.
d
, The laser beat frequency with a fibre laser during the ring resonance
blueshift sweep and redshift sweep. The vertical arrows mark the self-injection
locking range. The bottom plot is a calculation of asymmetric laser frequency-
locking range behaviour without thermal crosstalk for the bidirectional sweep.
The blue and red sections of the curve indicate stable and unstable branches,
respectively.
e
, The frequency noise of the laser output taken from the through
port and drop port of the 30-GHz ring resonator. Comparisons also show the
thermorefractive noise (TRN) of the 30-GHz-FSR ring resonator and β separation
line. The green curve shows the frequency noise of the SIL laser reported in ref.
32
and the grey dashed curve shows the noise f loor of the phase noise analyser
(PNA). PC, polarization controller; EDFA, erbium-doped fibre amplifier; ISO,
isolator; OSA, optical spectrum analyser; OSC, oscilloscope.
82
| Nature | Vol 620 | 10 August 2023
Article
results are summarized in Fig.
3d
. For the free-running state, the laser
enters regime II at an on-chip feedback level of −41 dB. This level of feed-
back can occur in typical waveguide couplers and splitters. As a result,
such feedback sensitivity puts a stringent requirement on the on-chip
or off-chip device design if isolators are removed. On the contrary,
self-injection locking with a high-
Q
cavity at both the through and the
drop ports sees a clear extended regime I. The critical feedback level
for the regime I boundary is increased to −14 dB and more than −10 dB,
respectively. We further increased the downstream on-chip feedback
level to the SIL drop port to −6.9 dB (limited by the chip-to-fibre cou-
pling loss) and observed a stable and constant laser linewidth—the
same as obtained below −50 dB downstream reflections (Fig.
3e
, top).
Such 27-dB and over-34-dB improvement in the feedback insensitiv
-
ity are equivalent to the effective isolation that optical isolators can
provide to maintain the laser coherence and thus enable isolator-free,
on-chip laser integration with downstream devices that introduce
ab
c
PD
PC
ESA
ISO
90
10
50
50
50
50
AOM
VOA
CIR
PC
PNA
Phase
Laser
Applied on-chip feedback strength (dB)
0
Frequency deviation (50 kHz div
–1
)
IV
I
SIL
through
0
–50
–45
–40
–35
–30
–25
–20
–15
IV
II
I
Free running
0
Time (s)
01
02
03
04
05
06
07
0
01
02
03
04
05
06
0
80
0
I
SIL
drop
Power
1
Free running
SIL
through
High-
Q
feedback
Downstream
reection
SIL drop
Laser
Laser
On-chip
Off-chip
10
3
10
0
10
2
10
4
10
6
10
8
10
4
10
4
10
5
10
6
10
7
10
5
10
6
10
7
Offset frequency (Hz)
Frequency noise (Hz
2
Hz
–1
)
–50.05 dB
–10.05 dB
–6.9 dB
On-chip feedback
d
e
1
Power
0
Time (s)
–50
0
50
Frequency deviation (kHz)
SIL drop
On-chip feedback: –6.9 dB
Ring
Cavity-loaded
Q
1
10
r1
(%)
100
R
= 25%
R
= 4%
R
= 1%
R
= 0.25%
Fig. 3 | Feedback insensitivity of the SIL laser.
a
, Schematic illustration of
the feedback inf luence for the laser working at the free-running state and self-
injection locking state. Under self-injection-locking conditions, both the
through port and the drop port are characterized.
b
, Experimental set-up for
the feedback sensitivity characterization.
c
, Calculation of the dependence of
the critical feedback level (the highest tolerable ref lection) for regime I
boundary (
f
r1
) on the cavity-loaded device. The backscatter (
R
) from the
ultrahigh-
Q
resonator also impacts the highest tolerable downstream
reflections.
d
, The laser spectral lineshape evolution recorded with an ESA by
self-heterodyning with an AOM for the free-running laser state (top), SIL
through-port output (middle) and SIL drop-port output (bottom). Different
feedback regimes are indicated and details of the regimes are covered in the
Supplementary Information.
e
, Frequency noise of the drop port at the SIL
condition under different on-chip feedback levels. The top panel shows the
recorded laser spectral lineshape evolution under a maximum of −6.9 dB
on-chip feedback. CIR, circulator.