Two-octave spanning supercontinuum
generation in stoichiometric silicon nitride
waveguides pumped at telecom wavelengths
MARCO A. G. PORCEL,
1,6
FLORIAN SCHEPERS,
2,6
JÖRN P.
EPPING,
1
TIM HELLWIG,
2
MARCEL HOEKMAN,
3
RENÉ G.
HEIDEMAN,
3
PETER J. M. VAN DER SLOT,
1,*
CHRIS J. LEE,
4
ROBERT SCHMIDT,
5
, RUDOLF BRATSCHITSCH,
5
CARSTEN
FALLNICH,
1,2
AND KLAUS-J. BOLLER
1
1
Laser Physics and Nonlinear Optics Group, MESA
+
Institute for Nanotechnology, University of Twente,
P.O. Box 217, Enschede 7500 AE, The Netherlands
2
Institute of Applied Physics, Westfälische Wilhelms-Universität, Corrensstrasse 2, 48149 Münster,
Germany
3
LioniX International B.V., PO Box 456, Enschede 7500 AL, The Netherlands
4
XUV Optics group, MESA
+
Institute for Nanotechnology, University of Twente, P.O. Box 217, Enschede
7500 AE, The Netherlands
5
Institute of Physics, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
6
These authors contributed equally to this work
*
p.j.m.vanderslot@utwente.nl
Abstract:
We demonstrate supercontinuum generation in stoichiometric silicon nitride (Si
3
N
4
in SiO
2
) integrated optical waveguides, pumped at telecommunication wavelengths. The pump
laser is a mode-locked erbium fiber laser at a wavelength of 1.56
µ
m with a pulse duration of
120 fs. With a waveguide-internal pulse energy of 1.4 nJ and a waveguide with 1.0
µ
m x 0.9
µ
m
cross section, designed for anomalous dispersion across the 1500 nm telecommunication range,
the output spectrum extends from the visible, at around 526 nm, up to the mid-infrared, at least
to 2.6
µ
m, the instrumental limit of our detection. This output spans more than 2.2 octaves (454
THz at the -30 dB level). The measured output spectra agree well with theoretical modeling based
on the generalized nonlinear Schrödinger equation. The infrared part of the supercontinuum
spectra shifts progressively towards the mid-infrared, well beyond 2.6
µ
m, by increasing the
width of the waveguides.
c
2017 Optical Society of America
OCIS codes:
(130.0130) Integrated optics; (190.4390) Nonlinear optics, integrated optics; (320.6629) Supercontinuum
generation; (190.7110) Ultrafast nonlinear optics.
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Vol. 25, No. 2 | 23 Jan 2017 | OPTICS EXPRESS 1542
#281693
Journal © 2017
http://dx.doi.org/10.1364/OE.25.001542
Received 28 Nov 2016; revised 6 Jan 2017; accepted 8 Jan 2017; published 19 Jan 2017
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1. Introduction
Supercontinuum generation (SCG), typically obtained with femtosecond pulses in photonic
crystal fibers [1], is a powerful method of providing extremely broadband spectra with full
spatial and temporal coherence [2, 3]. Such nonlinear optical generation is of high interest for
numerous applications, for instance in spectroscopy [4], with visible wavelengths in the life
sciences [5], or in precision metrology [2,6]. In addition, there is a growing relevance for coherent
on-chip generation of broadband light based on lasers in the telecommunication wavelength
range [7]. Envisioned applications include chip-sized frequency combs [8] or wideband integrated
microwave photonics [9, 10].
Using standard photonic crystal fibers, which typically requires interaction lengths of tens
of centimeters for efficient SCG, provides considerable design flexibility in their dispersive
and nonlinear properties [1]. However, the shot-to-shot coherence of the SC output decreases
with increasing pump pulse duration as well as with the interaction length [1, 11], particularly
in the presence of Raman scattering [11]. In contrast, the conversion of ultra-short pulses (<
100 fs) remains coherent in short fibers, but the conversion remains inefficient with a short
interaction length. Employing integrated optical waveguides on a chip provides the advantage
of tight mode confinement which strongly increases the nonlinear coefficient. This enables an
efficient conversion of ultrashort pulses also with short interaction lengths while preserving
coherence. For instance, pulses with up to a few hundred femtoseconds have been used for
efficient SCG in short optical waveguides while maintaining a high degree of coherence [12
–
14].
As a second advantage the integrated optical approach offers a route for high-volume and
low-cost fabrication, in particular when the waveguide platform is compatible with metal-oxide-
semiconductor (CMOS) fabrication facilities.
Lasers in the telecom range, due to their wide availability and maturity, have been employed
for SCG in a variety of photonic platforms that are compatible with CMOS fabrication facilities.
Examples are doped silica ridge waveguides using ultrashort pulses near 1300 nm and near
1550 nm [15, 16], buried silicon nitride waveguides using pulses near 1300 nm [17], or silicon
oxynitride waveguides with 1500 nm pulses [18]. The strongest nonlinear parameter and thus
SCG requiring low pulse energies can be found in smaller-bandgap or adjustable bandgap mate-
rials, e.g. chalcogenides [19, 20], silicon [21–23], and Si-enriched nitride glass [24]. Increasing
the nonlinearity via reducing the bandgap is, however, associated with increased nonlinear losses
at shorter wavelengths, such as via two-photon absorption.
A waveguide platform with highest relevance is stoichiometric SiN (Si
3
N
4
) grown with
low pressure chemical vapor deposition (LPCVD), because here a large variety of additional
functionalities is available with wafer scale fabrication. These functionalities are based on the
accessibility of an extremely wide wavelength range (from the blue, across the visible into the
infrared) and that, providing different types of waveguide cross sections [25] and tapers [26],
enable, e.g., polarization control, efficient fiber coupling, or bandwidth narrowing with hybrid-
integrated diode lasers [27, 28]. The intrinsic absorption and scattering is weak [29] which
offers exceptionally low propagation loss, below 0.001 dB/cm for standard waveguides [30]. The
latter is central for narrowband spectral filtering and optical delay lines as in quantum optical
systems [31], in microwave photonic filters [32, 33] or in programmable optical processors [34].
Finally, as Si
3
N
4
has negligible Raman gain, highly coherent supercontinua can be generated
using low pump energies [35]. So far in this platform SCG has been mainly investigated with
a relatively short pump wavelength around 1
µ
m, yielding spectral coverage mainly at shorter
wavelengths from the blue and across the visible into the near infrared [35–38] .
Here, we demonstrate a significant spectral widening of SCG towards the mid-infrared range.
Vol. 25, No. 2 | 23 Jan 2017 | OPTICS EXPRESS 1545
This was achieved with a longer pump wavelength in the 1.5
µ
m telecom range in combination
with dispersion engineering based on appropriate cross sections for the fabricated waveguides.
These were designed to shift anomalous dispersion towards longer wavelengths, including the
1.5
µ
m telecom wavelength range. The output spectra extend from about 526 nm to well beyond
2.6
µ
m, the limit of our detection instruments, thereby spanning at least 2.2 octaves, which is
more than 454 THz at the -30 dB level. The spectra broaden towards the mid-infrared with an
increasing width of the waveguides, in agreement with modelling.
2. Experimental setup
For enabling SCG with the available pump wavelength of 1560 nm, appropriate dimensions
of the waveguide core have to be chosen such that anomalous dispersion, i.e., negative group
velocity dispersion (GVD), is imposed broadly around that wavelength. Anomalous dispersion is
characterized by a positive value of the dispersion coefficient,
D(λ) = −λ/c · n
′′
(λ)
[39], where
λ
is the vacuum wavelength,
c
is the speed of light, and
n
′′
(λ)
is the second derivative of the
effective refractive index of the propagating mode with respect to the wavelength. We used a full
vectorial finite-element solver (Fimmwave, Photon Design) to calculate the effective refractive
index
n(λ)
as function of wavelength and then derived
D(λ)
. The two-dimensional step-index
profile used for modelling was based on the actual, somewhat rounded, shape of the waveguide
core as resulting from the fabrication process and obtained from scanning electron microscope
images of the waveguide cross section [36, 40]. The material dispersion of stoichiometric silicon
nitride and silica was taken from [41].
Figure 1 shows the calculated dispersion coefficient vs. the wavelength for waveguides of vari-
ous different widths between 0.7 and 1.3
µ
m, having the same height (0.9
µ
m). The calculations
imply that anomalous dispersion (
D >
0) can be obtained over wide spectral ranges including
the pump wavelength at 1560 nm if the waveguide core is chosen wider than approximately
0.8
µ
m. The calculations also show that increasing the waveguide width shifts the range of
anomalous dispersion noticeably towards the mid-infrared range, which is expected to shift SCG
to longer wavelengths as well.
Wavelength (nm)
D ( ps / (nm ∙ km) )
w = 0.7 µm
1
w = 1.3 µm
7
5001000150020002500
-200
-100
0
100
200
height
width
SiN
34
SiO
2
Fig. 1. Calculated dispersion coefficient,
D(λ)
, for the fundamental TM-mode in Si
3
N
4
waveguides with a height
h =
0
.
90
µ
m and widths varied from
w
1
=
0
.
70
µ
m to
w
7
=
1
.
3
µ
m in steps of 0.1
µ
m. Anomalous dispersion is present above the dashed zero-dispersion
line. The center wavelength of the pump laser at 1560 nm is indicated as red vertical line.
Figure 2 shows the experimental setup used for SCG based on a standard mode-locked erbium
fiber laser (Toptica, FemtoFiber Pro). The laser provided ultrashort pulses at a center wavelength
Vol. 25, No. 2 | 23 Jan 2017 | OPTICS EXPRESS 1546