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

Andres-Arroyo, Ana

Euser, Tijmen G.

Frosz, Michael H.

Köhler, Philipp

Ruskuc, Andrei

Russell, Philip S.

Weber, Marius A.

Caltech Authors - Main

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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 Koehlera,*, Andrei Ruskuca,b, Marius A. Webera, Michael H. Froszc, Ana Andres-Arroyoa,  
Philip St.J. Russellc, and Tijmen G. Eusera 
aNanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, JJ 
Thomson Ave, Cambridge CB3 0HE, UK, bT. J. Watson Laboratory of Applied Physics, California 
Institute of Technology, 1200 E California Blvd. MC 128-95, Pasadena, CA 91125, USA, cMax 
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
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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]. 
  
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oQo
aoo
 
 
 
3. MODE EXCITATION 
Efficient mode excitation was achieved with Laguerre-Gaussian beams (LG	୮(ℓ)). The electric field distribution in the 
focus of an LG beam is given by [22]: 
 ܧ୮(ℓ)(ݎ, ߶)~݁ି௥మ/௪మ ቀ௥௪ቁ
|ℓ| ܮ୮|ℓ| ቀଶ௥
మ
௪మቁ ݁௜థℓ, (1) 
where ℓ and p denote the azimuthal and radial order of the modes respectively, ܮ୮(|ℓ|) are the generalized Laguerre 
polynomials, ݎ and ߶ are polar coordinates in the focal plane and ݓ 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	ଷଵ mode (Fig. 2a) is well 
approximated by a superposition of LG	଴(ଷ) and LG	଴(ିଷ) 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 LP11 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 LP31 core mode in the kagomé PCF. (b) 
Measured 
intensity of an LG
(3) 
+ LG
(−3) 
beam profile. (c) Measured intensity profile of the excited LP31 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 5 10 15 20
0
0.2
0.4
0.6
1
0
0.2
0.4
0.6
0.8
1
1.2
Simulation
In
te
ns
ity
 / 
no
rm
al
iz
ed
1
0.5
0
10µm 10µm
Cam 1 Cam 3
10µm
In
te
ns
ity
 / 
no
rm
al
iz
ed
In
te
ns
ity
 / 
no
rm
al
iz
ed
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 LP11 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. 
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Excitation of higher-order modes in optofluidic hollow-core photonic crystal fiber

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Excitation of higher-order modes in optofluidic hollow-core photonic crystal fiber

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