This paper reports a long-period fiber gratings(LPFG) fabricated on a double-clad fiber (DCF) by the
CO2
laser point-by-point technique. The LPFG is tested by measuring temperature and refractive
index (RI) of different liquids. The results show that the LPFG inscribed on the DCF has no
measurable sensitivity to external RI but has a high sensitivity for temperature. The measured
temperature sensitivity is ~0.11 nm/oC, about 10 times higher than that of conventional FBGs. Such
LPFGs can be used as a modulator in fiber lasers or temperature/strain sensors in applications where
the surrounding RI is prone to variation, such as simultaneous measurement of the temperature and
RI of chemical reactions.
Long-period fiber gratings have been used for refractive index measurements under different conditions. Normally, this
kind of sensor is based on measuring resonance wavelength shift with respect to different refractive index environments.
As high order mode long period fiber gratings are attracting more attention, a new methodology based on intensity
measurement of turning points is introduced, which involves simple experiment setup and straightforward demodulation
process compared with wavelength shift based method. By using CO2 laser point by point irradiation method, high order
mode gratings working at turning point can be easily fabricated. This type of grating has a very high sensitive response
to surrounding refractive index, which can be used in chemical, medical and bio applications. In this paper, high
sensitive refractive index sensor is demonstrated based on high order mode using intensity measurement. Phase match
curve and couple mode theory are combined to analyze the intensity response to refractive index change at turning point
of LPFG. This sensor is also demonstrated as an effective refractive index based glucose sensor with a range from 0 to
40 mM concentration of glucose solution, which can fulfill the medical requirement.
Optical sensors are a common tool to measure the dissolved oxygen concentration in environmental, industrial and
medical areas. Much effort has been put on developing and using novel optical dyes and materials used as the
immobilization matrixes. A poly (ethylene glycol) (PEG)-rich hydrogel was used as a fluorophore matrix. For optical
sensor applications, this hydrogel was chemically anchored on negative-tone photopolymer SU-8 surface through a free
radical reaction in which 1-hydroxycyclohexyl phenyl ketone (HCPK) served as the surface bound photoinitiator.
Dissolved oxygen concentrations were detected based on the fluorescent intensity at emission wavelength of a
fluorophore, dichlorotris (1, 10-phenanthroline) ruthenium (II) hydrate 98%, toward dissolved oxygen molecules. The
normal characteristics of optical dissolved sensor were measured and recorded. All the results indicate the potential use
of patternable polymerized PEGDA membranes, which is chemically anchored to SU-8 surface, as an ideal candidate
matrix based on polymeric channel structures
Fourier transform spectroscopy has evolved over several decades into an analytic spectroscopic method which
application throughout the physical, chemical, and biological sciences. The conventional Fourier transform
spectrometer (FTS) is based on a scanning Michelson interferometer. To overcome the limitations induced by
mechanical scanning device in the Michelson interferometer, there are several types of FTS with non-scanning
system were presented. One of them utilizes the polarization element such as Wollaston prism or Savart plate. In
this paper the principle of a new FTS based on a Wollaston prism array is presented. Compared with the
conventional polarization FTS, it has higher spectral resolution and optical throughput. There are many factors
have influence on the fringe visibility of FTS. Here we focus on discussing the influence of the fringe visibility
affected by the direction of the polarization elements in this FTS. The fringe visibility of the interferogram is the
function which variables are the small angles departure of the theory polarization direction α, β. When the value of
the fringe visibility is given, the values of angle α, β, are obtained. This is helpful to the alignment of the polarizing
elements during the construction of polarization interferometer. The spectrum which uses the He-Ne laser as the
optical source is acquired and recovered through the Fourier-transform program. At present time the spectral
resolution of this system is 4 nm.
Due to the proper optical property and flexibility in the process development, an epoxy-based, high-aspect ratio
photoresist SU-8 is now attracting attention in optical sensing applications. Manipulation of the surface properties of SU-8 waveguides is critical to attach functional films such as chemically-sensitive layers. We describe a new integration
process to immobilize fluorescence molecules on SU-8 waveguide surface for application to intensity-based optical
chemical sensors. We use two polymers for this application. Spin-on, hydrophobic, photopatternable silicone is a
convenient material to contain fluorophore molecules and to pattern a photolithographically defined thin layer on the
surface of SU-8. We use fumed silica powders as an additive to uniformly disperse the fluorophores in the silicone
precursor. In general, additional processes are not critically required to promote the adhesion between the SU-8 and
silicone. The other material is polyethylene glycol diacrylate (PEGDA). Recently we demonstrated a novel photografting
method to modify the surface of SU-8 using a surface bound initiator to control its wettability. The activated surface is
then coated with a monomer precursor solution. Polymerization follows when the sample is exposed to UV irradiation,
resulting in a grafted PEGDA layer incorporating fluorophores within the hydrogel matrix. Since this method is based
the UV-based photografting reaction, it is possible to grow off photolithographically defined hydrogel patterns on the
waveguide structures. The resulting films will be viable integrated components in optical bioanalytical sensors. This is a
promising technique for integrated chemical sensors both for planar type waveguide and vertical type waveguide
chemical sensors.
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