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Excitation of higher-order modes in
optofluidic hollow-core photonic
crystal fiber
Philipp Köhler, Andrei Ruskuc, Marius A. Weber, Michael
H. Frosz, Ana Andres-Arroyo, et al.
Philipp Köhler, Andrei Ruskuc, Marius A. Weber, Michael H. Frosz, Ana
Andres-Arroyo, Philip St. J. Russell, Tijmen G. Euser, "Excitation of higher-
order modes in optofluidic hollow-core photonic crystal fiber," Proc. SPIE
10744, Laser Beam Shaping XVIII, 107440N (14 September 2018); doi:
10.1117/12.2320594
Event: SPIE Optical Engineering + Applications, 2018, San Diego, California,
United States
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Excitation of higher-order modes in optofluidic hollow-core photonic
crystal fiber
Philipp Koehler
a,*
, Andrei Ruskuc
a,b
, Marius A. Weber
a
, Michael H. Frosz
c
, Ana Andres-Arroyo
a
,
Philip St.J. Russell
c
, and Tijmen G. Euser
a
a
NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, JJ
Thomson Ave, Cambridge CB3 0HE, UK,
b
T. J. Watson Laboratory of Applied Physics, California
Institute of Technology, 1200 E California Blvd. MC 128-95, Pasadena, CA 91125, USA,
c
Max
Planck Institute for the Science of Light, Staudtstr. 2, 91058 Erlangen, Germany
ABSTRACT
Higher-order modes are controllably excited in water-filled kagomè-, bandgap-style, and simplified hollow-core
photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that
closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core.
Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomè HC-PCF and
match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors
and fiber-based optical manipulation.
Keywords: Photonic crystal fibers, Microstructured fibers, Spatial light modulators
1. INTRODUCTION
The controlled excitation of higher-order fiber modes has become an essential part in photonics research with a range of
interdisciplinary applications. For example, spatial light modulator (SLM)-based wavefront shaping techniques [1] have
enabled the controlled excitation of coherent mode superpositions in multimode fibers [2], with novel applications in
lensless endoscopic imaging [2]-[4] and fiber-based optical trapping [5]. In fiber communication systems, mode-division
multiplexing has been used to improve data transfer rates [6]-[9].
All this previous work aims to control the light field at the end-face of glass-core fibers. In hollow waveguides, on the
other hand, well-defined modal intensity distributions can be used to study light-matter interactions within the core. In
particular, hollow-core photonic crystal fiber (HC-PCF) has enabled the stable and low-loss transmission of modes along
microchannels [10]. The main classes of HC-PCF include bandgap-type HC-PCFs, in which a narrow transmission
window is supported by the formation of photonic bandgaps in the microstructured cladding, and kagomé- and simplified
HC-PCFs [11], whose broadband guidance mechanism relies on anti-resonant reflection. It has previously been shown
that spatial light modulators (SLM) can be used to dynamically change between different modes in air-filled hollow-core
photonic crystal fibers (HC-PCFs) [12], with applications in optical trapping [13], Raman amplification [14], telecoms
[15], and quantum optics [16].
Here we extend this work to liquid-filled HC-PCFs, where guidance properties are preserved by infiltrating both the core
and cladding channels [17]-[18]. Control over modal fields within these optofluidic waveguides would enable new fiber-
based sensing and optical manipulation approaches.
*pk428@cam.ac.uk; www.np.phy.cam.ac.uk/research-themes/optofluidics
Laser Beam Shaping XVIII, edited by Angela Dudley, Alexander V. Laskin, Proc. of SPIE Vol. 10744
107440N · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2320594
Proc. of SPIE Vol. 10744 107440N-1
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2. EXPERIMENTAL SETUP
We employ a method based on a spatial light modulation scheme recently presented by Flamm et al. [18] to controllably
excite higher order modes into the liquid-filled hollow-core photonic crystal fibers (HC-PCFs). This is achieved by
creating an intensity and phase distribution [20] that matches the HC-PCF mode and projecting it onto the fiber’s end
face. In Section A of Figure 2, light from a supercontinuum laser (NKT SuperK Compact, 450–2400 nm) is passed
through a variable bandpass filter (NKT SuperK Varia, 400–840 nm), expanded and linearly polarized. A 30 cm long
HC-PCF is mounted between two custom-made pressure cells (PCs), that are fitted with sapphire windows allowing for
unobstructed optical access (Section C). A phase-only SLM (Meadowlark P512-480-850-DVI-C512x512) with
broadband mirror coating shapes the beam and projects it in a 4-f configuration onto the fiber (Section B). Cam 2
measures the back-reflected light to help with the alignment process. With a microscope objective the transmitted mode
is imaged onto Cam 3 (Section C). Cam1, in Section D, is used to verify the SLM generated intensity profiles, see
examples in Figure 3.
Figure 1. Setup schematic. Section A: filtering, expansion, and polarization of the input beam. Section B: modulation
by phase-only SLM and projection onto the input-face of an HC-PCF. Section C: imaging of the end-face of the liquid-
filled HC-PCF, enclosed by two pressure cells (PC). Section D: verification of the intensity distribution projected onto
the HC-PCF. BE, beam expander; BS, beam splitter; Cam, camera; FM, flip mirror; Apert., aperture; P, polarizer; W,
waste. Figure reproduced from [21].
Proc. of SPIE Vol. 10744 107440N-2
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oQo
aoo
3. MODE EXCITATION
Efficient mode excitation was achieved with Laguerre-Gaussian beams (LG
(ℓ)
). The electric field distribution in the
focus of an LG beam is given by [22]:
(ℓ)
(,)~
/
|
ℓ
|
|ℓ|
ℓ
, (1)
where ℓ and p denote the azimuthal and radial order of the modes respectively,
(|ℓ|)
are the generalized Laguerre
polynomials, and are polar coordinates in the focal plane and is the beam waist. To excite a specific mode, pairs of
LG beams with an appropriate relative phase were chosen. For example, the predicted LP
mode (Fig. 2a) is well
approximated by a superposition of LG
()
and LG
()
beams (Fig. 2b). Mode-excitation experiments were performed in
three different water-filled HC-PCFs including the bandgap HC-PCF, the kagomé HC-PCF, and the simplified HC-PCF.
Figure 3 shows the measured intensity distribution of an LP
11
mode excitation in each one of these fibers. Additional
excited modes and a more detailed analysis can be found in [21].
Figure 2. Mode excitation example: (a) Simulated intensity profile of a LP
31
core mode in the kagomé PCF. (b)
Measured
intensity of an LG
(3)
+ LG
(−3)
beam profile. (c) Measured intensity profile of the excited LP
31
fiber mode. Radial- (d)
and azimuthal (e) sections along the dashed curves in (a–c). Figure reproduced from [21].
Φ / radians
0.8
Simulation
Measured LG Beam
Measured Mode
0
510
15
20
0
0.2
0.4
0.6
1
0
0.2
0.4
0.6
0.8
1
1.2
Simulation
Intensity / normalized
1
0.5
0
10µm
10µm
Cam 1 Cam 3
10µm
Intensity / normalized
Intensity / normalized
r / µm
π/3 2π/3 π 4π/3 5π/3 2π0
(a) (b) (c)
(d)
(e)
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r
/-
/''
Figure 3. Mode excitation in fibre: Measured intensity profile of a LP
11
core mode (d)-(f) in the photonic bandgap HC-
PCF, kagomé HC-PCF and simplified HC-PCF (a)-(c).
4. CONCLUSION AND OUTLOOK
We demonstrate a spatial light modulation setup that can be used to efficiently excite higher-order modes in liquid-filled
HC-PCFs. The setup was tested on three different types of water-filled HC-PCFs (bandgap, kagomé, and simplified).
While the observed modes were relatively pure and launch efficiencies high (10–20%), further improvements could be
made by correcting for aberrations in the optical system and using a more robust hologram optimization routine.
The results provide a framework for new spatially-resolved sensing and optical manipulation experiments in liquid-filled
hollow-core PCF. Measurements using different spatial modes would enable the probing of chemicals at varying
distances from the core wall and thus provide a direct measurement of surface effects and microscale diffusive transport,
both of which are rate-limiting factors in HC-PCF microreactors [23] and flow-chemistry in general. In optical
manipulation studies, superpositions of higher-order modes can be used to create reconfigurable 3-D intensity patterns
within the hollow core [13] that could be used to trap, transport, and separate micro- and nanoparticles along the fluid
channel.
REFERENCES
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32, 2309–2311 (2007).
[2] T. Čižmár and K. Dholakia, “Exploiting multimode waveguides for pure fibre-based imaging,” Nat. Commun.
3, 1027 (2012).
[3] Y. Choi, C. Yoon, M. Kim, T. D. Yang, C. Fang-Yen, R. R. Dasari, K. J. Lee, and W. Choi, “Scanner-Free and
Wide-Field Endoscopic Imaging by Using a Single Multimode Optical Fiber,” Phys. Rev. Lett. 109, 203901
(2012).
[4] L. V. Amitonova, A. Descloux, J. Petschulat, M. H. Frosz, G. Ahmed, F. Babic, X. Jiang, A. P. Mosk, P. St.J.
Russell, and P. W. H. Pinkse, “High-resolution wavefront shaping with a photonic crystal fiber for multimode
fiber imaging,” Opt. Lett. 41, 497–500 (2016).
Kagome HC-PCFPhotonic bandgap HC-PCF Simplified HC-PCF
(c)
(b)
10µm
20µm
10µm
50µm 20µm
10µm
(d)
(e) (f)
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![Figure 1. Setup schematic. Section A: filtering, expansion, and polarization of the input beam. Section B: modulation by phase-only SLM and projection onto the input-face of an HC-PCF. Section C: imaging of the end-face of the liquidfilled HC-PCF, enclosed by two pressure cells (PC). Section D: verification of the intensity distribution projected onto the HC-PCF. BE, beam expander; BS, beam splitter; Cam, camera; FM, flip mirror; Apert., aperture; P, polarizer; W, waste. Figure reproduced from [21].](/figures/figure-1-setup-schematic-section-a-filtering-expansion-and-1u6qgvvf.webp)
