Whispering gallery mode microsphere resonator
integrated inside a microstructured optical fiber
Kyriaki Kosma,1 Gianluigi Zito,1 Kay Schuster2 and Stavros Pissadakis1,*
1Foundation for Research and Technology-Hellas (FORTH), Institute of Electronic Structure and Laser (IESL),
P.O. Box 1315, Heraklion, 71 110, Greece
2Institute of Photonic Technology Jena, Albert-Einstein-Str. 9, 07745 Jena, Germany
*Corresponding author: email@example.com
Received Month X, XXXX; revised Month X, XXXX; accepted Month X,
XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX
A compact and robust scheme for broadband excitation of whispering gallery mode (WGM) resonances into a microsphere is
demonstrated. A polymer microsphere (10 μm) is encapsulated into the capillary of microstructured optical fiber, in direct
contact with the guiding core. Such a configuration allows efficient and reproducible excitation of the in-MOF-microsphere
resonator that is characterized by two launch/collection schemes: core-input/scattering-output, and sphere-input/core-output.
The latter allows an excitation of the microsphere WGMs externally to the fiber. Numerically calculated WGM spectra are in
agreement with experiments. Q factors in the range of 103 are typically measured. © 2012 Optical Society of America
OCIS Codes: 060.4005, 230.4555, 060.2340, 060.2370
A considerable amount of work has been focused on the
excitation of various micro-resonator structures (micro-
discs, -spheres and toroids) for observing resonant light
trapping based on whispering gallery mode (WGM)
generation . Aiming at photonic [2, 3] and bio-chemical
[4, 5] sensing applications, excitation techniques based on
evanescent field coupling using optical waveguides, fibers
and prisms have been extensively reported in studying
the fundamental physical mechanisms underlying light
confinement into those microstructures [6-8].
In microsphere resonators, WGMs are excited using
tapered optical fibers positioned at evanescent field
proximity, utilizing benchtop micropositioners. High Q-
factor (105-107) devices based on large size parameters
πd/λ (achieved, e.g., with microspheres of diameter d in
the range ~102-103 μm) have been demonstrated,
targeting at ultra-low molarity bio-chemical detection, or
for developing high finesse lasers [9-14]. Efficiency of the
modal excitation, and related Q-factor of the resonance,
critically depends upon the dielectric properties of the
resonator, its geometry, modal orders, the launching
condition and polarization of the input optical field. Also,
optimization of the system is typically required for the
particular spectral range of interest [2, 9]. Alternatively,
microspheres have been positioned/attached on the end
face of standard  and microstructured optical fibers
(MOFs)  leading to Q-factor resonances of 5x106 and
5x102, respectively. However, most of the WGM devices
above require ultra-stable coupling conditions that are
dependent upon strict alignment and handling procedures
between the optical stimulator and the resonator.
In this Letter, we report the realization of a compact
and robust WGM resonator by using a microstructured
optical fiber as the encapsulating and excitation system of
a polymer microsphere, boosting the potential resonator
applicability and expanding the lab-in-fiber protocol .
In addition, contamination issues that are deleterious for
the device performance are addressed through such a
photonic design [5, 18]. In spite of lowering the Q factor
ceiling by utilizing polystyrene spheres with typical
diameter ≤10.5μm, such a size microspheres lead to
resonance spectra more sensitive to the geometry and
refractive index of the environment . In that optical
configuration, a germanium-doped silica core MOF,
suspended within three hollow channels, is controllably
infiltrated with a polystyrene microsphere (PM)
embedded in contact with the “mercedes-shaped” core; the
latter has a thickness of ~2.3 μm in the center and ~0.25
μm at the inner capillary boundaries [see Fig. 1(a)-(b)].
Through the long tail evanescent field, 0th guiding mode of
the collapsed core MOF used, light is coupled to/and from
the microsphere, leading to the WGM excitation and
power exchange between these two optical components.
Given the particular cladding geometry of the fiber (that
is monomode at the 1.5μm), the light injected into the ≈2
μm small core of the MOF allows an efficient evanescent
coupling with the higher index PM, forming a MOF-
microsphere resonator. This work intends to demonstrate
a photonic device simple to realize and interrogate, while
scaling down cumbersome optical bench WGM excitation
setups into an integrated MOF-platform, addressing
stability issues and enhancing versatility.
A polystyrene microsphere (d=10.43 μm, Polymer
Laboratories), having complex
ñ=1.5915+i4·10-4 at 589 nm  was encapsulated in one
of the MOF hollow channel (HC) (maximum width ~10.5
μm) by capillarity action. Selection and positioning of the
sphere on the cleaved end-face of the fiber was achieved
using a glass micropipette; immersion into ethanol
allowed a controllable displacement of the microsphere
along the HC by suction [Fig. 1(a)]. Typically, the
microsphere rests 3-4 cm away from the fibre end. After a
few minutes of low temperature annealing ethanol was
totally evaporated leaving the PM embedded into the HC
at the desired position as evident in Fig. 1(b), in which a
scanning electron microscopy (SEM) image of the MOF-
microsphere resonator is shown. Since the diameter of the
MOF channels and the microsphere are almost identical,
the last is in direct contact both with the fiber core and the
cladding side of the capillary.
Fig. 1. (a) PM suspended in ethanol inside an HC of the MOF.
(b) SEM scan of the cleaved end-face of the mercedes-shaped
MOF encapsulating a PM. (c) Schematic representation of the in-
MOF-resonator excitation and collection setup: in C/S mode the
beam of the SCLS injected from the microscope objective (A)
follows the green path; in S/C mode the beam is injected from the
objective (B) and follows the pink path.
Prior, we define the TE (S) and TM (P) polarization
states with electric field in the xy- and xz-plane of Fig.1(c).
The WGM modal numbers are set as following: azimuthal
mode number l (i.e. the number of wavelengths confined
within a round trip inside the sphere); radial mode
number n (i.e. the number of radial field maxima); and
polar mode number m . To investigate the response of
the system, the resonant radiation trapped in the
microsphere was observed by means of two different
excitation/collection schemes: the core-input/scattering-
output (C/S) coupling mode, and the sphere-input/core-
output (S/C) coupling mode [see Fig. 1(c)].
The C/S coupling scheme, exploited the fiber core to
excite the whispering gallery modes of the encapsulated
micro-resonator, while the photons scattered from the
latter were collected by a microscope objective positioned
at 90° with respect to the fiber axis [Fig. 1(c)]. The light
from a super-continuum laser source (SCLS), with
spectral range 400-2000 nm (NKT SuperK Compact), was
coupled to the MOF core by a 25× objective, indicated as
(A) in Fig.1(c); whereas the PM scattering spectrum was
collected by a second 25× microscope objective (B). Always,
the fundamental guiding mode was excited using the
objective (A); this was verified at the MOF output using
the objective (C) [see Fig.1(c)]. The signal collected at point
(B), was filtered by a Glan-Thompson polarizer; then
coupled to an optical spectrum analyzer (OSA), using a
multimode fiber (MMF). Due to coupling losses and low
spectral density of the SCLS, the spectral measurements
were obtained using a 0.5-nm resolution (0.1-nm for the
individual mode measurements).
Examples of experimental spectra of the light scattered
at 90° from the micro-resonator, measured in C/S scheme,
are shown in Fig. 2. The spectra display series of discrete
peaks, referring to the resonant modes that are excited
and circulated inside the spherical cavity. The broadband
WGM signature, extending from visible to near-infrared
bands, exhibits extinction peaks up to 10 dB and Q-factors
resolved up to ~2200. The present Q-factor results are
greater than those presented with MOF end-face
excitation in reference , thanks to the coupling scheme
of the in-fiber resonator architecture.
Fig. 2 Upper and middle panels: Intensity scattered by the PM
and collected in C/S mode, optimized in four spectral regions,
with TM mode identification numbers. Bottom panel: (left)
measurement of a high Q and spectrally resolved resonance
obtained using 0.1-nm resolution in the OSA, while fitted with a
Lorentzian function; (right) near-infrared spectrum with
simulation results for TE (blue) and TM (red) polarization
excitation. All intensity scales are linear.
presented are in good agreement with those predicted in
simulations. The resonance pattern is characterized by a
strong peak at the fundamental l=|m|, n=1 (having the
highest coupling efficiency) and a few peaks of higher
radial order n=2, 3.. (l-shifted according to the n-th zero of
the Airy function , i.e. the corresponding effective
diameter). The fine structure related to the m-degeneracy
is also resolved because of shape irregularities of the
microsphere used and coupling condition as usual in such
a kind of systems: a pressure or temperature fluctuation
δd~20 nm in the PM diameter has been calculated to
produce a shift δλ~1 nm of the peak position in the NIR
range. In Fig. 2, numerical simulations based on Finite
Difference Time Domain (FDTD) method, are also shown,
for TE and TM polarizations. The relative peak
intensities, as well as, small shifts of the expected
resonance from experimental value and related Q-factor,
depend on the particular effective coupling achieved
between the core and the microsphere. Ambient
temperature fluctuations, cladding induced eccentricity
and different thermal expansion coefficients between
silica fiber and PM can lead to deviations from the
theoretical model. Although the photonic configuration
presented here did not allow varying the gap between the
spherical resonator and the fiber core, a fine tuning of the
coupling was still achievable by modifying the launching
parameters at the input (A) of the fiber core [see Fig.1(a), .
experimental wavelength resonance data
Fig. 3 (Left panel) Light intensity scattered by the PM and
collected in C/S mode optimized in the near-infrared spectral
range, for TE (blue) and TM (red) polarizations. (Right panel)
Spectrum of selected TE peak, obtained with 0.1-nm resolution in
the OSA. Intensity scales are linear.
polarizations obtained at longer wavelengths, where
polarization dependencies are more prominent, appear in
Fig. 3. The intrinsic polarization sensitivity of the MOF
used, together with the differential WGM resonance for
the two polarizations  underline this behavior. For the
specific wavelength band a significant shift between the
two polarizations is observed (≥10nm); smaller TE/TM
experimental spectra for TE/TM
shifts were measured at shorter wavelength bands.
Fig. 4 (Left panel) Experimental fiber output spectrum
resulting from the S/C mode coupling (black). (Right panel) same
data are compared with simulation (red) for a smaller
wavelength range. Intensity scale is linear.
The second launch/collection scheme, namely the S/C
mode, resulted from coupling the SCLS into the spherical
resonator through the microscope objective (B) [Fig. 1(c)],
and collecting the light at the fibre core output through
the 60× microscope objective (C), again coupled through a
MMF to the OSA. For eliminating heating effects induced
by the SCLS beam, a bandstop infrared absorption filter
was placed before the Glan-Thompson polarizer,
attenuating all wavelengths above ~800 nm [not shown in
Fig. 1(c)]. Fig. 4 shows an example of transmission
spectrum recorded in S/C coupling, with a characteristic
WGM signature being observed. Alignment was more
critical now, leading to a decrease of the Q-factor, typically
lower than ~500. In Fig. 4, numerical calculation of the
same spectral pattern is shown, revealing a broadening of
the resonant peak structures. Coupling through the
cylindrical, astigmatic cladding induces a degeneracy loss,
exciting a greater number of modes in the sphere than the
C/S mode. Out-cladding coupling, even though it expands
the potential functionality of the resonator, also excites all
Mie resonances of the PM, however, at lower efficiencies,
pointing out the importance of the coupling conditions.
By comparing the C/S coupling mode to the S/C one, one
can propose that the first device architecture is more
suitable for real field sensing applications in which a high
stability and reproducibility operation is needed. In such a
type of designs the microsphere can be i.e. bio-
functionalised for performing targeted sensing. This
resonator/fiber coupler, thanks to the easy integration of
the microsphere directly inside the hollow channel of the
MOF, can also support multi-analyte sensing through the
same device, by infiltrating microspheres of different
functionalisation or opto-geometrical characteristics.
A stable and robust in-fiber microsphere/resonator
coupler based on whispering gallery modes has been
integrated nature of this MOF-based resonator can lead to
the development of novel photonic bio-chemical sensors.
The authors acknowledge partial support from the
COST action TD1001 OFSeSa.
Q-factors ~2.2x103. The
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