We report a compact in-fiber Mach-Zehnder interferometer (MZI) made along a hollow-core photonic bandgap fiber
(HC-PBF). The MZI uses a long period grating (LPG) and an offset-splice joint (OSJ) which act as beam splitter and
combiner respectively. The LPG was produced by use of a high frequency pulsed CO2 laser, and the OSJ was made in
serial with the LPG by using a commercial fusion splicer. The interference is between the fundamental core mode (FCM)
and a high-order core mode (HOCM). The proposed interferometer was experimentally tested for temperature and strain
measurements, and the sensitivities of the interference fringe dip wavelength to temperature and strain are 107.5 pm/
(°C-m) and -1.24 pm/με, respectively.
We investigate the strain sensitivity of long period gratings (LPGs) fabricated in hollow-core photonic bandgap fibers
(HC-PBFs). The LPGs are fabricated by periodically deforming the air-holes along the PBFs by use of a CO2 laser.
Resonant couplings between the LP01 and LP11 core modes results in two highly polarization-dependant loss dips in the
transmission spectrum. The sensitivity of the resonant wavelength to strain was found to vary from -1.48 to -2.4 pm/με when the grating pitch was changed from 225 to 175 μm. Theoretical investigation suggests that the size and shape of the
core can have significant influence on the strain sensitivity.
Selective opening and closing of air-holes in photonic crystal fibers (PCFs) are realized by using a femtosecond infrared
laser and a CO2 laser. By heating/tapering the PCFs while pressurizing those opened holes, the birefringent properties of
the PCFs can be modified. High birefringence of 3.6×10-4 and 1.5×10-3 were obtained by post-processing commercial
LMA-10 and HC-1550-02 fibers.
We investigate the polarization dependent characteristic of an acousto-optic filter in photonic crystal fiber and find
polarization dependence is strong. By inserting the filter into a fiber loop mirror, the polarization dependence is
eliminated effectively, and the polarization dependent loss is reduced from ~4.0 dB to <0.35 dB.
A long period grating (LPG) is a longitudinal periodic optical structure that drives couplings from the
fundamental core mode into phase-matched co-propagating cladding modes of an optical fiber and a series
of attenuation dips are formed in the transmission spectrum . LPGs have been applied as photonic
sensors to detect external perturbations including temperature, strain, bending and surrounding refractive
index, by monitoring the spectral shifts of the resonant dips . LPGs are conventionally fabricated by UV-light
exposure to induce periodic refractive-index variation of 10-5 ~ 10-4 in the fiber core. Such an LPG is
regarded as weak perturbation to the fiber and the mode coupling process has been described by the wellknown
coupled mode theory (CMT) .
In addition to the UV-inscription technique, stronger LPGs can also be formed by introducing refractive
index/geometry modulation by use of CO2-laser irradiation, arc discharge, and periodic tapering [4-6].
Photonic crystal fibers (PCFs), which contain a two-dimensional array of air holes in their claddings,
provide an extra-dimension for LPG-inscription through periodic deformation of the air-holes in the
cladding . However, the conventional CMT may not provide accurate description to these strong LPGs
because of the significant modification of the mode fields and refractive indexes over the modulated
regions. In this paper, the mode coupling process in a strong LPG inscribed in a PCF is quantitatively
analyzed based on the coupled local-mode theory. The analysis offers a physical insight and a better
understanding over the energy transfers in the LPGs. Based on the theory, a general phase-matching
condition for LPG is presented, which accurately determines the resonant wavelengths λres.
This paper reports some of our recent work on in-line devices based on air-silica microstructrue optical fibers.
These devices are fabricated by use of a CO2 laser/a femtosecond infrared laser and include strong long period
gratings in index-guiding fibers and air-core photonic bandgap fibers, in-fiber polarizers, polarimeters, and modal
interferometers. Applications of such devices for strain, temperature, directional bend, twist, and gas sensing are
A directional bend sensor is demonstrated based on a long period grating fabricated in photonic crystal fiber by
introducing geometric deformations in the cladding air holes with a CO2 laser. A bend sensitivity of 2.26 nm/m-1 is
achieved within the range of -5~+5 m-1.
We demonstrated the fabrication of nonadiabatic tapers in air-core photonic bandgap fibers. In-fiber Mach-Zehnder
interferometers were formed by utilizing such tapers and experimentally demonstrated for strain and temperature
This paper reports the development of a fiber-optic gas detection system capable of detecting three types of
dissolved fault gases in oil-filled power transformers or equipment. The system is based on absorption spectroscopy and
the target gases include acetylene (C2H2), methane (CH4) and ethylene (C2H4). Low-cost multi-pass sensor heads using
fiber coupled micro-optic cells are employed for which the interaction length is up to 4m. Also, reference gas cells made
of photonic bandgap (PBG) fiber are implemented. The minimum detectable gas concentrations for methane, acetylene
and ethylene are 5ppm, 2ppm and 50ppm respectively.
Long period gratings in hollow-core photonic-bandgap fibers were fabricated by use of a pulsed CO2 laser. The resonant
wavelengths of these gratings are sensitive to strain but insensitive to temperature, bend and external refractive index.
A broadband, compact in-fiber polarizer was fabricated by using a pulsed CO2 laser to modify the air-holes along oneside
of a hollow-core photonic bandgap fiber. The polarizer has a length of 3 to 6mm and exhibits a polarization
extinction ratio of better than 20 dB over a wavelength range of 100nm around 1550nm.
A series of low-contract photonic band-gap (PBG) fibers were fabricated by filling the holes of a commercial air-silica
hollow-core PBG fiber with different refractive index liquids. The PBGs and the transmission characteristics of these
fibers were investigated theoretically and experimentally. An increase in the refractive index of liquid filling the holes
causes blue-shift of the PBG and a narrow down of the PBG width, which may be exploited for sensitive refractive index
We report our recently research on holey optical fiber sensors, including gas detection using hollow-core and solid-core
silica holey fibers, two-mode holey fibers and their applications in strain and temperature measurements, long period
gratings inscribed on holey fibers and their strain and temperature characteristics.
In a long period grating (LPG) made on a silica-based single material photonic crystal fiber (PCF), the effect of material dispersion on the resonance wavelength of the LPG is negligible. The resonance wavelength, the period and length of the LPG, and the diameter and pitch of the air -hole lattice of the PCF are found to obey a scaling law. Simulations show that the resonance wavelength has a non-monotonic dependence on the grating period and, for a particular grating period, there could exist dual resonance wavelengths and hence double transmission dips due to phase matching between the fundamental core mode and a cladding mode simultaneously at two wavelengths. This phenomenon may be explored for novel devices and sensor applications.
Index-guiding photonic crystal fibers (PCFs) with appropriate structural parameters support the fundamental and second order modes over a practically infinite wavelength range. The potential applications of such PCFs are discussed.