of 6
Research Article
Vol. 9, No. 3 / March 2022 /
Optica
303
Intense optical parametric amplification in
dispersion-engineered nanophotonic lithium
niobate waveguides
Luis Ledezma,
1
,
2
,
Ryoto Sekine,
1
,
Qiushi Guo,
1
,
Rajveer Nehra,
1
Saman Jahani,
1
AND
Alireza Marandi
1
,
*
1
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125, USA
2
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
*Corresponding author: marandi@caltech.edu
Received 2 September 2021; revised 25 February 2022; accepted 1 March 2022; published 14 March 2022
Strong amplification in integrated photonics is one of the most desired optical functionalities for computing, communi-
cations, sensing, and quantum information processing. Semiconductor gain and cubic nonlinearities, such as four-wave
mixing and stimulated Raman and Brillouin scattering, have been among the most studied amplification mechanisms on
chip. Alternatively, material platforms with strong quadratic nonlinearities promise numerous advantages with respect
to gain and bandwidth, among which nanophotonic lithium niobate is one of the most promising candidates. Here, we
combine quasi-phase matching with dispersion engineering in nanophotonic lithium niobate waveguides and achieve
intense optical parametric amplification. We measure a broadband phase-sensitive on-chip amplification larger than
50 dB/cm in a 6-mm-long waveguide. We further confirm high gain operation in the degenerate and nondegenerate
regimes by amplifying vacuum fluctuations to macroscopic levels, with on-chip gains exceeding 100 dB/cm over 600 nm
of bandwidth around 2
μ
m. Our results unlock new possibilities for on-chip few-cycle nonlinear optics, mid-infrared
photonics, and quantum photonics.
© 2022 Optical Society of America under the terms of the OSA Open Access Publishing
Agreement
https://doi.org/10.1364/OPTICA.442332
1. INTRODUCTION
Amplification is an important element of a wide range of optical
systems, from computing [1] and sensing [2] to quantum infor-
mation processing [3] and communications [4]. In integrated
photonics, achieving intense amplification remains an impor-
tant challenge. In silicon-based platforms, significant attention
has been focused on cubic nonlinearities to realize amplification
through four-wave mixing (FWM) [5,6], stimulated Raman
scattering (SRS) [7], and stimulated Brillouin scattering (SBS)
[8]. Despite recent promising advances, the weak nature of these
nonlinearities and the adverse effects of other competing nonlin-
earities hamper the amount of gain and bandwidths associated with
these mechanisms. Another option providing gain on integrated
platforms is the semiconductor optical amplifier (SOA). SOAs
have evolved in the past decades as one of the leading optical gain
mechanisms [9,10], and heterogeneous integration of III-V SOAs
with other platforms, especially silicon, has been one of the most
active research directions in integrated photonics [10]. However,
their limited bandwidth and integration challenges hinder their
utilization in several applications, such as those that require access-
ing gain in multiple places on a chip. Furthermore, semiconductor
gain is not phase-sensitive, limiting its use in quantum and com-
munication applications that require noiseless amplification,
e.g., processing of quantum microcombs [11] and few-cycle
squeezed vacuum [12]. Hence, an integrated platform with a native
gain mechanism that enables intense and phase-sensitive optical
amplification of ultrashort pulses can address several of the current
challenges in photonics.
Quadratic nonlinearities provide an alternative path for achiev-
ing strong optical amplification through three-wave mixing
[13,14]. Such processes have been extensively used in bulk optical
systems, leading to amplification at wavelengths where other gain
mechanisms are not easily available [15,16]. Recently, integrated
photonic platforms with strong quadratic nonlinearities have
attracted significant attention, since they can provide a range of
functionalities unavailable in other platforms [17–20]. Examples
of these processes include second harmonic and supercontinuum
generation [21,22], electro-optic modulation [23,24], quadratic
parametric oscillators [25,26], and bright sources of entangled
photons [27]. Despite the recent significant progress, realization of
intense optical amplification in quadratically nonlinear integrated
photonics has remained elusive.
In integrated photonics, strong quadratic nonlinear inter-
actions have been enabled by tight spatial confinement of the
waveguide modes and the possibility of providing momentum con-
servation through modal [28] or quasi-phase matching [21,22].
Further enhancement has also been achieved by utilization of
2334-2536/22/030303-06 Journal © 2022 Optica Publishing Group
Research Article
Vol. 9, No. 3 / March 2022 /
Optica
304
appropriate resonators [25,26]; however, resonant dynamics
associated with the cavity lifetime are typically not appropriate
for amplification in many applications, as they limit the gain
bandwidth.
In this work, we present an integrated, high-gain, broadband,
traveling-wave, optical parametric amplifier based on quadratic
nonlinearities. We show phase-sensitive amplification by operat-
ing the amplifier at degeneracy. The large parametric gain of our
device is enough to amplify quantum fluctuations to macroscopic
levels, therefore allowing the amplifier to function as an optical
parametric generator of infrared radiation. Our design strategy is
based on quasi-phase matching combined with spatiotemporal
confinement of pulses in dispersion-engineered lithium niobate
waveguides, a combination that is not easily available on other
nonlinear photonic platforms.
2. DEVICE DESIGN AND FABRICATION
We focus on OPA at degeneracy through three-wave mixing in
a
χ
(
2
)
waveguide [Fig. 1(a)]. As shown in Fig. 1(b), for efficient
short-pulse OPA, negligible group velocity dispersion (GVD)
at the signal and pump wavelengths (
ω
s
and
ω
p
) are required to
preserve the temporal confinement of these pulses and hence their
high peak intensities along the waveguide. Additionally, in quad-
ratic parametric processes, the group velocity mismatch (GVM)
between the pump and signal frequencies needs to be minimized so
that both pulses travel together along the waveguide, maximizing
their parametric interaction. The effects of GVD and GVM on the
OPA gain spectrum are shown in Fig. 1(c) for a 6-mm-long wave-
guide for three different waveguide geometries. These numerical
simulations confirm the importance of dispersion engineering for
maximizing the gain and bandwidth of OPA around degeneracy.
We design our waveguides for degenerate OPA of signal wave-
lengths around 2
μ
m, with a pump centered at 1045 nm. The
GVD and GVM we obtain are marked as “dispersion-engineered”
in Fig. 1(c), where we also show the corresponding curves for
nonzero GVD and nonzero GVM cases. For a 35-fs-long signal
pulse, the optimized waveguide has a dispersion length of more
than 30 mm at 2090 nm, and a walk-off length between the pump
(1045 nm) and the signal (2090 nm) of almost 100 mm. In com-
parison, other cases in Fig. 1(c) correspond to a waveguide with
nonzero GVD, which has a dispersion length of 2 mm at 2090 nm,
and a nonzero GVM waveguide with a 1 mm walk-off length.
Beyond temporal confinement, nanophotonic waveguides also
enable subwavelength spatial confinement. Figure 1(d) shows the
profiles of the fundamental quasi-TE modes of the waveguide
for the pump and signal wavelengths. The similarity of both field
distributions produces a large modal overlap and a strong nonlin-
ear interaction (see Supplement 1, Section 1), leading to intense
amplification.
With this dispersion-engineered waveguide, where pump and
signal pulses copropagate at the same group velocity with negligible
linear distortion, one can approximate the parametric process with
a continuous-wave model [22] (see also Supplement 1, Section 3).
At degeneracy, the pump frequency is twice the signal frequency,
leading to phase-sensitive amplification. A signal with the cor-
rect phase with respect to the pump [Fig. 1(a)] is amplified by a
factor of exp
(
2
g L
)
in a device of length
L
. The gain parameter
is
g
=
η
P
pump
(1
k
/
2
)
2
, where
P
pump
is the pump power,
η
is the nonlinear efficiency, and
1
k
is the phase mismatch after
quasi-phase matching (
1
k
=
β
p
2
β
s
2
π/3
), with a constant
poling period
3
. When the relative phase between signal and
pump is changed by
π
, the device transitions from a degenerate
OPA to a second-harmonic generator with energy flowing from the
Fig. 1.
Parametric amplification in dispersion-engineered PPLN waveguides. (a) In degenerate optical parametric amplification (OPA) through three-
wave mixing in a
χ
(
2
)
medium, energy is transferred from the pump at
ω
p
to signal at
ω
p
/
2, providing amplification for the signal. When the relative phase
between pump and signal changes by
π
, the flow of energy reverses, resulting in deamplification of the signal. (b) In a PPLN waveguide, GVD leads to pulse
temporal spreading with a decrease in peak power and gain, while GVM causes temporal walk-off between the pump and signal pulses, reducing their inter-
action. Hence, engineering the waveguides for low GVD and GVM is necessary to maximize the OPA performance. (c) Simulated relative gain spectrum
for the three dispersion cases shown in (b) in a 6-mm-long waveguide with 75-fs pump pulses, along with the simulated GVM (with respect to the pump at
1045 nm) and GVD. The dispersion-engineered lithium niobate waveguide (blue trace) exhibits low GVM between the pump at 1045 nm and the signal
around 2090 nm, and low GVD for both wavelengths, and it has a top width of 1700 nm, an etch depth of 350 nm, and total thin-film thickness of 700 nm.
The orange trace represents a waveguide with low GVM but large GVD (900-nm top width, 680-nm thickness, 420-nm etch depth), while the green trace
is for a waveguide with low GVD but large GVM (3-
μ
m top width, 750-nm thickness, 150-nm etch depth). (d) Electric field profiles of the fundamental
quasi-TE modes for the dispersion-engineered waveguide at the pump and signal wavelengths.