We report an integrated whispering gallery mode microresonator–based sensor probe for refractive index sensing. The probe was made by sealing a borosilicate glass microsphere into a thin-wall glass capillary pigtailed with a multimode optical fiber. The intensities of the resonant peaks were found decreasing exponentially (linearly in a log scale) with the increasing refractive index of the medium surrounding the capillary. The sensing capability of the integrated probe was tested using sucrose solutions of different concentrations and the resolution was estimated to be about 2.5×10−5 in the index range of 1.3458 to 1.3847. The integrated sensor probe may prove useful in many chemical and biological sensing applications where highly sensitive refractive index monitoring is needed.
Optical fiber interferometers (OFIs) have been extensively utilized for precise measurements of various
physical/chemical quantities (e.g., temperature, strain, pressure, rotation, refractive index, etc.). However, the random
change of polarization states along the optical fibers and the strong dependence on the materials and geometries of the
optical waveguides are problematic for acquiring high quality interference signal. Meanwhile, difficulty in multiplexing
has always been a bottleneck on the application scopes of OFIs. Here, we present a sensing concept of optical carrier
based microwave interferometry (OCMI) by reading optical interferometric sensors in microwave domain. It combines
the advantages from both optics and microwave. The low oscillation frequency of the microwave can hardly distinguish
the optical differences from both modal and polarization dispersion making it insensitive to the optical
waveguides/materials. The phase information of the microwave can be unambiguitly resolved so that it has potential in
fully distributed sensing. The OCMI concept has been implemented in different types of interferometers (i.e., Michelson,
Mach-Zehnder, Fabry-Perot) among different optical waveguides (i.e., singlemode, multimode, and sapphire fibers) with
excellent signal-to-noise ratio (SNR) and low polarization dependence. A spatially continuous distributed strain sensing
has been demonstrated.
Optical microresonators have been proven as an effective means for sensitive chemical sensors development. The
changes in refractive index near the resonator surface lead to the effective refractive index change and thus a shift at
certain resonance wavelength. The high quality (Q) whispering gallery modes (WGMs) contributed by the rotationally
symmetric structures will interact with the local circumstances through the evanescent field. The high sensitivity in
detection was achieved by the long photon lifetime of the high-Q resonator (thus the long light-environment interaction
path).
In this paper, we present our recent research on using fiber pigtailed capillary coupler for WGM resonator excitation and
its sensing applications. Capillary tube with wall thickness of several microns was used as the waveguide. The PMMA
microsphere and porous glass microsphere (PGM) were integrated with the etched capillary tube for different sensing
purposes. The Q-factors and free spectrum ranges (FSR) of different types of microspheres were measured by coupling
light into the microsphere using novel fiber pigtailed capillary coupler. Chemical vapor at different concentrations were
tested using PGM microresonator. This alignment free structure provides a new sensing probe based on WGM resonator
concept.
A U-shaped optical fiber inline microchannel was fabricated by femtosecond laser irradiation and subsequent selective
chemical wet etching. A high quality micro-cavity embedded inside the channel was obtained to construct a Fabry-Perot
interferometer (FPI). A fringe visibility of 20 dB in spectrum domain was achieved. High temperature survivability of
this micro device was also demonstrated. The proposed assembly-free optical fiber inline interferometer is attractive for
sensing applications in high-temperature harsh environments.
We report a reflection-based phase-shifted long period fiber grating (PS-LPFG) and demonstrate its capability for simultaneous measurement of temperature and external reflective index (RI). The sensor device comprises a grating directly written by CO2 laser and silver-coated end face. A π-shifted LPFG is presented with two attenuation bands through its reflection spectrum. These two bands have different sensitivity towards temperature and external RI that can be used for simultaneous measurement of the two variables. The experimental results show that this probe-type PS-LPFG performs well in terms of linearity and sensitivity.
Optical microresonators have been proven effective for developing sensitive chemical and biological sensors by
monitoring the changes in refractive index or mass near the resonator surface. The rotationally symmetric structures
support high quality (Q) whispering gallery modes (WGMs) that interact with the local environment through the
evanescent field. The long photon lifetime of the high-Q resonator (thus the long light-material interaction path) is the
key reason that a microresonator can achieve very high sensitivity in detection. In this paper, we present our recent
research on using porous wall hollow glass microsphere (PW-HGM) as an optical microresonator for chemical vapor
detection. The diameter of the PW-HGM ranges from 10μm to 100μm. The wall thickness is about 2μm and the pore
size is about 20nm. The Q-factors and free spectrum ranges (FSR) of PW-HGMs were measured by coupling light into
the PW-HGM using a single mode fiber taper. Various types of chemical vapors were used to characterize the PW-HGM
resonator. The resonant wavelength shift was measured as a function of vapor concentration. Comparisons between a
PW-HGM and a solid glass microsphere indicated that a PW-HGM can effectively adsorb vapor molecules into its nanosized
pores, providing a direct and long light-material interaction path for significant sensitivity enhancement for
chemical vapor detection.
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