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2–10 µm Mid-Infrared Fiber-Based Supercontinuum
Laser Source: Experiment and Simulation
Sebastien Venck, François St-Hilaire, L Brilland, Amar Nath Ghosh, Radwan
Chahal, Céline Caillaud, Marcello Meneghetti, J Troles, Franck Joulain,
Solenn Cozic, et al.
To cite this version:
Sebastien Venck, François St-Hilaire, L Brilland, Amar Nath Ghosh, Radwan Chahal, et al.. 2–10
µm Mid-Infrared Fiber-Based Supercontinuum Laser Source: Experiment and Simulation. Laser &
Photonics Reviews, 2020. �hal-03023809�
2-10 µm Mid-Infrared All-Fiber Supercontinuum Laser Source:
Experiment and Simulation
Sébastien Venck
1
, François St-Hilaire
2,6
, Laurent Brilland
1
, Amar N. Ghosh
2
, Radwan Chahal
1
,
Céline Caillaud
1
, Marcello Meneghetti
3
, Johann Troles
3
, Franck Joulain
4
, Solenn Cozic
4
, Samuel
Poulain
4
, Guillaume Huss
5
, Martin Rochette
6
, John Dudley
2
, and Thibaut Sylvestre
∗2
1
SelenOptics, Campus de Beaulieu, Rennes, France
2
Institut FEMTO-ST, CNRS UMR 6174, Université Bourgogne Franche-Comté, Besançon,
France
3
Université de Rennes, CNRS, ISCR-UMR 6226, Rennes, France
4
Le Verre Fluoré, Campus Kerlann, Bruz, France
5
LEUKOS, 37 rue Henri Giffard, Limoges, France
6
Department of Electrical and Computer Engineering, McGill University, Montréal, Québec,
Canada
December 20, 2019
Abstract
Mid-infrared supercontinuum (SC) sources in the 2 to 20 µm molecular fingerprint region are in
high demand for a wide range of applications including optical coherence tomography, remote sensing,
molecular spectroscopy, and hyperspectral imaging. In this work, we investigate mid-IR SC generation
in a cascaded silica-ZBLAN-chalcogenide fiber system directly pumped with a commercially-available
460-ps pulsed fiber laser operating in the telecommunications window at 1.55 µm. This all-fiber system
is shown to generate a flat broadband mid-IR SC covering the entire range from 2 to 10 µm with several
tens of mW of output power. This technique paves the way for cheaper, practical, and robust broadband
SC sources in the mid-IR without the requirement of mid-infrared pump sources or Thulium-doped
fiber amplifiers. We also describe a fully-realistic numerical model used to simulate the nonlinear
pulse propagation through the cascaded fiber system and we use our numerical results to discuss
the physical processes underlying the spectral broadening in the cascaded system. We conclude with
recommendations to optimize the current cascaded systems based on our simulation results.
∗
corresp ond ing author: thibaut.sylvestre@univ-fcomte.fr
1
1 Introduction
Fiber-based supercontinuum (SC) sources have become enormously useful in the last decade in wide range
of industrial and scientific applications [1, 2]. New uses are constantly emerging due to their unique
properties that combine high brightness, multi-octave frequency bandwidth, fiber delivery and single-
mode output. Applications include optical coherence tomography (OCT), material processing, chemical
sensing, gas monitoring, broadband imaging, absorption spectroscopy. Currently, there is a significant
research effort focused on extending the wavelength coverage towards the mid-Infrared (Mid-IR) in the 2
to 20 µm molecular fingerprint region [3–18]. Various soft glasses based on chalcogenide (As
2
S
3
, As
2
Se
3
,
GeAsSe) [3,8], tellurite (TeO
2
) [19], telluride (GeTe, GeAsTeSe) [20, 21], heavy-metal oxide (PbO-Bi
2
O
3
-
Ga
2
O
3
-SiO
2
-CdO) [10] and ZBLAN (ZrF
4
–BaF
2
–LaF
3
–AlF
3
–NaF) [23, 24, 26], have been used for
drawing highly nonlinear infrared fibers, and experiments have shown efficient mid-IR SC generation up
to 14 µm in chalcogenide optical fibers and up to 16 µm in telluride fibers [3,21]. However, most of these
mid-IR SC sources have been demonstrated using bulky mid-IR pump sources such as optical parametric
oscillators (OPO) and amplifiers (OPA). Mid-IR fiber lasers and cascaded fiber systems have recently
emerged as very attractive and promising solutions for practical and commercial applications [25–33, 35].
These all-fiber systems indeed open routes to practical, table-top and robust mid-IR supercontinuum lasers
with high spectral power density. Of particular interest are cascaded fiber systems as they are pumped
by standard pulsed fiber lasers at telecommunication wavelength. In cascaded SC generation, an initial
pulsed fiber laser at 1550 nm is progressively red-shifted in a cascade of silica and soft-glass fibers, enabling
a stepwise extension towards the Mid-IR. From a fundamental point of view, the overall objective is to
strongly enhance the soliton self-frequency shift (SSFS) using dispersion-tailored highly nonlinear fiber
segments to push forward the SC generation far in the mid-IR [31].
Among the most advanced all-fiber systems, Martinez et al. recently demonstrated a mid-IR SC from
2 to 11 µm with 417 mW on-time average power by concatenating solid-core ZBLAN, arsenic sulfide,
and arsenic selenide fibers, pumped by a master oscillator power amplifier and three thulium-doped fiber
amplifier stages [29,30]. Hudson et al. combined a 2.9 µm ultrafast fiber laser based on holmium with an
environmentally stable, polymer-protected chalcogenide fiber taper. By launching femtosecond, 4.2 kW
peak power pulses into the As
2
Se
3
/As
2
S
3
tapered fiber, they demonstrated a SC spectrum spanning from
1.8 to 9.5 µm with an average power of more than 30 mW [27]. C.R. Petersen et al. also demonstrated
in 2016 Mid-IR SC generation beyond 7 µm using a silica-fluoride-chalcogenide fiber cascade pumped by
a 1.55 µm seed laser and a thulium-doped fiber amplifier. By pumping a commercial Ge
10
As
22
Se
68
-glass
photonic crystal fiber with 135 mW of the pump continuum from 3.5- 4.4 µm, they obtained a continuum
up to 7.2 µm with a total output power of 54.5 mW, and 3.7 mW above 4.5 µm [33]. Commercially
available mid-IR SC source with an extended bandwidth up to 10 µm and with 50 mW output power is
now on sale at Norblis [34].
In this paper, we demonstrate all mid-IR all-fiber SC generation from to 2 to 10 µm in a cascaded silica-
ZBLAN-As
2
Se
3
chalcogenide fiber system directly pumped with a commercially-available 500-ps pulsed
fiber laser at 1.55 µm and without any fiber amplifier stage. We provide the details of the experimental
cascaded fiber system and discuss the parameters as dispersion and nonlinear coefficients used for optimal
spectral broadening. We further report a fully-realistic numerical model based on cascaded generalized
nonlinear Schrödinger equations with an adaptive step size method. The model includes all the linear and
nonlinear responses of the fibers, the infrared filtering function, and the effective mode area variation across
the full SC spectrum. We then use our numerical results to discuss the physical processes underlying the
spectral broadening in the cascaded system as modulation instability (MI), soliton fission and dispersive
wave generation [1,2,36,37]. We conclude with recommendations to optimize the current cascaded systems
based on our simulation results.
2
2 Experimental Setup and Results
A diagram of the experimental setup for cascaded mid-IR SC generation is shown in Figure 1(a). It
consists of a concatenation of three different optical fibers including a short 20 cm-long single-mode silica
fiber (SMF-28), a 25m-long ZBLAN fiber, and a 9-m long chalcogenide-glass photonic crystal fiber, all
commercially available. Detailed specifications for each fiber segment are given in the next section including
all parameters used in numerical simulations. This cascaded fiber arrangement was directly pumped by a
1550.6 nm Erbium-doped fiber laser delivering 460 ps pulse train at 90 kHz repetition rate and with an
average output power of 750 mW. For a pulse energy of 8.33 µJ and a pulse duration around 460 ps, the
corresponding pulse peak power is around 18 kW. Light coupling between each fiber was achieved using
high-numerical aperture aspheric lenses matching the fiber numerical apertures and their effective mode
areas. A key element in the cascaded fiber system is a mid-IR long pass filter (LPF) placed in between
the ZBLAN and the chalcogenide fibers. This filter that filters out all the wavelengths below 1.9 µm.
This prevents from two-photon absorption (TPA) and optical damage in the chalcogenide fiber, enabling
a better long-term stability of the mid-IR SC source. As a long-pass filter, we used a Germanium (Ge)
Ar-coated broadband window (Thorlabs WG91050) in between two aspheric lenses. The generated SC
spectra have been recorded using a mid-IR optical spectrometer including a monochromator (ORIEL 7240)
and a highly sensitive Hg-Cd-Te detector. The chalcogenide fiber output end was further connectorized
using an FC/PC connector for pratical applications.
Figure 1: (a) Experimental setup for mid-infrared SC generation in a cascaded silica-ZBLAN-chalcogenide
optical fiber system. (b) Experimental SC spectra at the ZBLAN fiber output (blue) and after the long-
pass filter (red). (c) Experimental SC spectrum at the chalcogenide fiber output (yellow).
We provide experimental data for the spectra measured at three key points in the cascaded fiber setup
(A,B,C). The first spectrum, shown in Figure 1(b) (light blue), was measured directly at the output of the
ZBLAN fiber at point A, before the long-pass infrared filter. The supercontinuum spans from 0.7 µm up
to 4.1 µm. The second spectrum, also shown in figure 1(b) (dark blue), was measured after the filter at
point B. This is the filtered IR spectrum injected into the chalcogenide fiber. The final SC spectrum at the
output of the chalcogenide fiber is shown in figure 1(c) in yellow. As can be seen, the full SC spans from 2
3
µm up to 9.8 µm with a relatively smooth and flat bandwidth. The measured spectrum however shows a
decrease in spectral intensity for shorter wavelengths and significant modulations in intensity (especially
from 2 to 4 µm). Our simulations will show that those are not entirely a result of SC dynamics, but
presumably artefacts from the wavelength sensitivity of the spectrometers used for measurement. The
measured average power at the output is 16 mW, which corresponds to roughly 2% of the pump’s average
power (750 mW). Significant losses occur in the cascaded system, mainly from the free-space optics between
the ZBLAN fiber and the chalcogenide fiber, including Fresnel reflections and coupling losses due to mode
field diameter mismatch, aspheric lenses, cleaving imperfections, and optical misalignment. Despite this
low conversion efficiency, the brightness of this source of a few microwatt per nanometer is sufficient for
practical applications.
3 Numerical Method
To simulate nonlinear pulse propagation in the cascaded fiber system, we used the generalized nonlinear
Schrödinger equation (GNLSE) and solved the propagation equation numerically with the split-step Fourier
method (SSFM) [2, 37] combined with an adaptive step size [44]. The GNLSE can be written in the
following form:
∂A(z, T )
∂z
= −
α
2
A + i
∞
X
k=2
i
k
β
k
k!
∂
k
∂T
k
A + iγ
1 + iτ
0
∂
∂T
A · R(T ) ~ |A|
2
(1)
This equation models the evolution of the complex pulse envelope A(z, T ) as it propagates inside an optical
fiber in a frame of reference moving with the group velocity of the pulse: T = t − β
1
z. The first term on
the right models optical losses with the linear loss coefficient α. The second term models dispersion with
a Taylor expansion of the propagation constant β(ω), where β
k
=
∂
k
β(ω)
∂ω
k
. The third term models Kerr
nonlinearity, including the Raman response of the material and a first-order correction for the frequency
dependence of the nonlinear parameter γ, γ(ω) =
ωn
2
(ω)
cA
eff
(ω)
. The first-order correction is referred to as the
shock term and is characterized by a time-scale τ
0
, which can include the frequency-dependence of A
eff
,
n
2
, and n
eff
(see [43] for more details).
τ
0
=
1
ω
0
−
1
n
eff
(ω)
dn
eff
dω
ω
0
−
1
A
eff
(ω)
dA
eff
dω
ω
0
+
1
n
2
(ω)
dn
2
dω
ω
0
(2)
The nonlinear response of the fiber is modeled with a convolution (denoted by ~) of the nonlinear response
function R(T) and the pulse power profile |A(z, T )|
2
. R(T ) is commonly divided into an instantaneous
electronic response (Kerr) and a delayed molecular response (Raman), and reads as R(T) = (1−f
R
)δ(T )+
f
R
h
R
(T ), where f
R
is the fractional contribution of the delayed Raman response, δ(T ) is the Dirac delta
function that models the instantaneous Kerr response, and h
R
(T ) is the delayed Raman response function.
R(T ) is normalized such that
R
∞
−∞
R(T )dT = 1. The Raman response function h
R
(T ) is modeled using
two characteristic times related to phonon dynamics in the material, τ
1
and τ
2
(see Ref [37] for more
details):
h
R
(T ) = (τ
−2
2
+ τ
−2
2
)τ
1
exp(−T/τ
2
) sin(T /τ
1
) (3)
We will use Eq. (3) for both the silica and chalcogenide fibers (See Table 1 for parameters). However,
as the ZBLAN fiber has a dual-peak Raman gain spectrum, we will another model based on Eq. 5. A
crucial step in modeling nonlinear pulse propagation in a cascaded fiber system is defining the longitudinal
step size h. To accurately model the effects of nonlinearity and dispersion, h must be much smaller than
both the dispersion length and the nonlinear length, defined as, L
NL
=
1
γP
0
and L
D
=
T
2
0
|β
2
|
, respectively,
4
![Figure 2: ZBLAN step-index optical fiber specifications: (a) Core (blue) and cladding (Red) refractive indices versus wavelength. (b) Computed dispersion (blue curve) and experimental data (Red dots). (c) Simulated (red) and experimental (blue) effective mode area. (d) Loss spectrum: blue: Measured data, red: data from [41]](/figures/figure-2-zblan-step-index-optical-fiber-specifications-a-3a12go0h.webp)
