In order to solve for the mode intensity distributions in a photonic crystal fiber (PCF) cross section and the propagation constant for the design of fiber bandpass filters, we numerically analyze the modal distributions of the fundamental core mode and different cladding modes. Based on the simulation results, we also experimentally demonstrate a simple fabrication of bandpass filters inscribed on the PCF by inserting a π-phase shift in a 12-period long-period grating (LPG). Two rejection bands with greater than 18 dB isolation and an ultra-wide band of 85.3 nm are achieved. The phase-shifted PCF-LPGs are fabricated using a CO2 laser with point-by-point focused pulses. The proposed fiber bandpass filter is compact and is not influenced by temperature effects.
The emerging concept of structural health management relies on extensive onboard diagnostic sensors that can provide near real-time information about the state of a structure so that informed prognostic assessment can be made of the continuing reliability of the structure. In this paper, we will discuss two types of sensing platforms that can provide valuable information about the state of a structure: 1D fiber-optic sensors and 2D thin-film sensors. Both fiber-optic and thin film sensors are easily integrated with structures, and can offer local and/or distributed sensing capabilities. Parameters that can be sensed include: static and dynamic strain, acoustic emission, vibration, corrosion products, moisture ingression etc. We will first describe some recent developments in dynamic strain sensing using optical fiber Bragg grating (FBG) sensors. Applications to detection of acoustic emission and impact will be described. In the area of chemical sensing, we will describe a nanofilm-coated photonic crystal fiber (PCF) long-period grating (LPG) sensing platform. PCF-LPG sensors can be designed to provide greater interaction between the analyte of interest and the light propagating in the fiber, thereby increasing the sensitivity of detection. Applications to humidity sensing will be described. Finally, 2D thin-film sensors on polymer substrates will be discussed. One type of sensor we have been fabricating is based on reduced graphene oxide for large-area chemical sensing applications. It is expected that these 1D and 2D sensing platforms will form part of a suite of sensors that can provide diagnostic structural health information.
Fiber-optic gas sensing techniques are commonly based on the recognition of a wide range of chemical species from characteristic absorption, fluorescence or Raman-scattering spectra signatures. By tuning over the vibrational lines of species in the path of laser beam, tunable diode laser gas sensors measure signal spectroscopic intensity, gas concentration, and other properties. However, they have limitations of bulk architecture, small change of signal on top of large background, and low sensitivity of direct absorption. Here we report the fabrication and optical measurements of tunable Er-doped fiber ring laser absorption spectroscopic sensor featuring a gas cell that is a segment of photonic crystal fiber (PCF) with a long-period grating (LPG) inscribed. The tunable laser beam is coupled into the cladding of the PCF by the LPG where the gas in air holes absorbs light. The light travels along the PCF cladding and reflects at the end of the fiber where a silver film is coated as a mirror at one end facet. The light propagates back within cladding and passes through the gas one more time thus increasing the interaction length. This light is finally recoupled into the fiber core for intensity measurement. The proposed fiber gas sensors have been experimentally used for ammonia (NH3) concentration detection. They show excellent sensitivity and selectivity, and are minimally affected by temperature and/or humidity changes. The sensors using PCF-LPG gas cell are simple to fabricate, cost-effective, and are deployed for a variety of applications not possible using conventional optical fibers.
Fiber-optic accelerometers have attracted great attention in recent years due to the fact that they have many
advantages over electrical counterparts because all-fiber accelerometers have the capabilities for multiplexing to
reduce cabling and to transmit signals over a long distance. They are also immune to electromagnetic
interference. We propose and develop a compact and robust photonic crystal fiber (PCF) Mach-Zehnder
interferometer (MZI) that can be implemented as an accelerometer for measurements of vibration and
displacement. To excite core mode to couple out with cladding modes, two long-period gratings (LPGs) with
identical transmission spectra are needed to be written in an endless single-mode PCF using a CO2 laser. The
first LPG can couple a part of core mode to several cladding modes. After the light beams travel at different
speeds over a certain length of the core and cladding, the cladding modes will be recoupled back to the core
when they meet the second LPG, resulting in interference between the core mode and cladding modes. Dynamic
strain is introduced to the PCF-MZI fiber segment that is bonded onto a spring-mass system. The shift of
interference fringe can be measured by a photodetector, and the transformed analog voltage signal is
proportional to the acceleration of the sensor head. Based on simulations of the PCF-MZI accelerometer, we can
get a sensitivity of ~ 0.08 nm/g which is comparable with fiber Bragg grating (FBG) accelerometers. The
proposed accelerometer has a capability of temperature insensitivity; therefore, no thermal-compensation
scheme is required. Experimental results indicate that the PCF-MZI accelerometer may be a good candidate
sensor for applications in civil engineering infrastructure and aeronautical platforms.
Using long-period gratings (LPGs) inscribed in an endless single-mode photonic crystal fiber (PCF) and coating
nanostructure film into air channels in the PCF cladding with modal transition of the LPG, we have developed a
fiber-optic sensing platform for detection of chemicals. PCF-LPG possesses extremely high sensitivity to the change
in refractive index and chemical selectivity by localizing binding and/or absorption events in analyte solution. In this
work, we numerically and experimentally investigate the behaviors of modal transition in the PCF-LPG where the
air channels of PCF cladding are azimuthally coated with two types of nanostructure polymers as primary and
secondary coatings by electrostatic self-assembly (ESA) deposition technique. The primary coating does not affect
PCF-LPG parameters such as grating resonance wavelengths and its intensity that can be used for sensing, but it
increases the sensitivity to refractive index of chemical analytes in the air channels. The secondary coating is for
selective absorption of analyte molecules of interest. Those two coatings significantly modify the cladding mode
distribution of PCF-LPG and enhance the evanescent wave interaction with the external environment, leading to a
highly sensitive and selective chemical sensor. The integrated sensor has potential in a variety of applications,
especially for nano-liter scale measurement in situ. The functional nanostructure films which respond to different
parameters can be introduced into the air channels of the PCF-LPGs as transducers with chemical selectivity. In this
paper, we demonstrate a fiber-optic humidity sensor with the proposed nanofilm-coated PCF-LPG for detection of
corrosion in civil infrastructural health monitoring.
Long-period gratings (LPGs) have shown their significant promising applications in sensors owing to the attractive
features that they posses such as small size, immunity for electromagnetic interference, geometric versatility,
multiplexing capability, and resistance to corrosive and hazardous environments. Recent researches have revealed that
LPGs written on the standard optical fibers could be used as a powerful sensing platform for structural health
monitoring. In this work, we inscribe LPGs into SMF-28 optical fiber by focused-beam CO2 laser, demonstrating as a
refractive index sensor for nondestructive chemical detections in the civil infrastructures. Although evanescent-field
based LPG sensors have been applied in quantitatively monitoring chemical analytes including moisture, chloride, and
corrosion by-product, etc., the sensitivity, selectivity, and response time as well as thermo-stability of such sensors are
still the issues for some special purposes. In order to improve those characteristics of the sensors, we propose two types
of nano-film to be coated in grating region by electrostatic self-assembly (ESA) deposition processing. The primary
coating does not affect on LPG transmission parameters such as resonance wavelength and its intensity that can be used
for sensing, but it increases the sensitivity to refractive index change of surrounding material. The secondary coating is
for selectively absorption of analyte molecule of interest. Response time of the nanofilm-coated LPG sensor is dependent
on the analyte absorption and de-absorption rates as well as the thicknesses of the coating materials, which is also
investigated. Multi-channel sensor system is being designed to monitor different analytes simultaneously, which is
continuing to further explore the monitoring of structural health conditions through in situ measurements of corrosion in
the concrete structures.
The array of axially aligned air channels and the robust waveguide characteristics of index-guiding photonic crystal
fibers (IG-PCFs) integrated with long-period gratings (LPGs) make them a powerful platform for chemical sensing and
detection. Compared to their conventional all-solid fiber counterpart, the IG-PCFs are a particularly attractive sensing
device as they are both a waveguide and a vapor/aqueous transmission cell, permitting light intensity-analyte interaction
over long path length without the removal of fiber cladding. While the fundamental core-mode in the IG-PCF has been
utilized for evanescent field based sensing, there exist two inherent limitations: (1) only short distance extended by
evanescent waves from the guiding core to the surrounding PCF cladding air channels to restrict the probing of an
analyte only in the inner most ring of the air channels in cladding, and (2) less than 1% power of the core-mode overlap
with the surrounding air channels leading to weak light intensity-analyte interactions due to the localization of the coremode
in the fiber core area. Should a cladding-mode with maximum overlap in air channels be excited by an LPG, it
would fundamentally increase the evanescent field sensitivity. In this work, we present the simulation for the mode
properties of selected IG-PCF for optimization of mode field distribution and light power overlap with air channels in
fiber cladding. The numerical calculation reveals that if the optimized cladding-mode is selectively coupled, the
evanescent wave overlap (at wavelength of 1550 nm) with cladding air channels of the round and hexagonal structures can be increased from 0.11% and 0.13% up to 4.01% and 6.54%, respectively.
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