In this paper, we summarize our recent research progresses on the understanding, design, fabrication, characterization of various photonic sensors for energy, defense, environmental, biomedical and industry applications. Femtosecond laser processing/ablation of various glass materials (fused silica, doped silica, sapphire, etc.) will be discussed towards the goal of one-step fabrication of novel photonic sensors and new enabling photonic devices. A number of new photonic devices and sensors will be presented.
This paper presents an approach for simultaneous measurement of temperature and pressure using miniaturized fiber inline sensors. The approach utilizes the cascaded optical fiber inline intrinsic Fabry–Perot interferometer and extrinsic Fabry–Perot interferometer as temperature and pressure sensing elements, respectively. A CO2 laser was used to create a loss between them to balance their reflection power levels. The multiplexed signals were demodulated using a Fast Fourier transform-based wavelength tracking method. Experimental results showed that the sensing system could measure temperature and pressure unambiguously in a pressure range of 0 to 6.895×105 Pa and a temperature range from 20°C to 700°C.
We demonstrate a compact all-fiber probe for a common path optical coherence tomography (CPOCT) system.
By forming a focusing lens directly on the tip of an optical fiber, a compact fiber probe could be constructed. The
microlens is produced by forming a droplet with UV-curing adhesive on the cleaved tip of multimode fiber. It fulfills
two functions acting as both the reference plane and the imaging lens. To simultaneously achieve a relative long working
distance and a good lateral resolution, we employed a large core size multimode fiber. A working distance of 280μm,
and a transverse resolution of 14μm were achieved with the implemented MMF lensed fiber. The performance of the
CPOCT system with the proposed MMF lensed fiber is presented by showing the OCT images of an onion tissue as
biological sample.
Due to earthquake effects, buildings often experience large strains, leading to progressive collapses. Monitoring and
assessing the large strain condition of critical buildings is of paramount importance to post-earthquake responses and
evacuations in earthquake-prone regions. However, few monitoring system can work under such harsh environments.
For their unique attributes such as compactness, immunity to electromagnetic interference and capability integrated
within various types of structures and materials, optical fiber sensors are especially attractive for quasi-distributed
strain sensing purposes in harsh environments. Nevertheless, the dynamic range of strain measurements of an optical
sensor is limited by the elasticity of the optical fiber. In this paper, a quasi-distributed optical fiber sensor network
based on extrinsic Fabry-Perot interferometer (EFPI) and long-period fiber grating (LPFG) sensors for both large strain
and high temperature measurements has been developed. The sensor network combined several inline EFPIs and
LPFGs by various couplers. Each EFPI sensor in the sensor network system has the capacity of large strain
measurement up to 12% and each LPFG sensor here has a temperature measurement range of up to 700°C. To obtain
strain and temperature information for multiple locations more efficiently, a hybrid LPFG/EFPI optical fiber sensor
based sensor network system has been studied in this paper. Experimental results demonstrate that the proposed quasi-distributed
optical fiber sensor network system is capable for both large strain and high temperature measurements.
Therefore, the proposed optical fiber sensor network system can be applied to monitor the quasi-distributed strain of
civil infrastructure in harsh environments.
We demonstrate experimentally the light coupling between two types of optical fiber devices, including the long period
fiber gratings (LPFGs) and fiber tapers. Optical power transfer is achieved through evanescent field coupling between
the cladding modes. The output spectra from the LPFG couplers exhibit band-pass/band-rejection pattern, which could
be used as add/drop filters. The output spectra from the fiber taper couplers show an interference pattern similar to a
Mach-Zehnder interferometer. By fixing the fiber devices in a capillary glass tube, we demonstrated a miniaturized
coupler for displacement sensing.
Long period fiber grating (LPFG) has been attracted much attention for use in optical sensing applications and optical
communication systems. Compared with a regular LPFG, the phase shift long period fiber grating (PS-LPFG) has shown
certain unique advantages such as higher sensitivity and potentials for simultaneous measurements of multiple
parameters. This paper presents the fabrication and characterization of PS-LPFG by CO2 laser point-by-point
irradiations. We emphasis on the difference between LPFG and PS-LPFG as optical sensors for measurement of strain,
temperature and refractive index.
Long-period fiber grating (LPFG) sensors have been demonstrated to exhibit negative and positive strain sensitivity
coefficients with cladding modes of LP05 and LP07, respectively. Their temperature sensitivity remains positive for both
cladding modes. Based on the unique properties of CO2 laser-induced gratings, two designs of LPFG sensors have been
presented in this paper for simultaneous strain and temperature measurement. One double-LPFG sensor integrates two
separate groups of gratings in series, corresponding to two cladding mode measurements such as LP05 and LP07. It can
measure strain and temperature simultaneously with a measurement error of less than 2%. However, the spacer between
two tandem gratings can only survive in low temperature environments. The other single-LPFG sensor uses the
measurements with two cladding modes on one group of gratings. Under various disturbances, different intrinsic cladding modes of the single LPFG, such as LP05 and LP07, are tracked simultaneously for strain and temperature discrimination. The single-LPFG sensor can measure strains in high temperature up to 700°C.
This paper summarizes our recent research progresses in developing optical fiber harsh
environment sensors for various high temperature harsh environment sensing applications
such as monitoring of the operating conditions in a coal-fired power plant and in-situ
detection of key gas components in coal-derived syngas. The sensors described in this paper
include a miniaturized inline fiber Fabry-Perot interferometer (FPI) fabricated by one-step fs
laser micromachining, a long period fiber grating (LPFG) and a fiber inline core-cladding
mode interferometer (CMMI) fabricated by controlled CO2 laser irradiations. Their operating
principles, fabrication methods, and applications for measurement of various physical and
chemical parameters in a high temperature and high pressure coexisting harsh environment
are presented.
There is a range of ways to couple light in a single mode fiber (SMF) from core mode to cladding modes, which can be
applied in some fiber sensors. Recently, a very simple method using CO2 laser irradiation is put forward. By coupling
core mode to cladding mode in the first irritation point and re-coupling in the second one, in-line Mach-Zehnder
interferometer (MFI) and Michelson interferometer(MI) sensors have be demonstrated.
To understand the mechanism underneath this coupling phenomenon, several parameters (laser power, laser lasting time,
etc) tests are investigated. With bigger laser power and longer lasting time, one can obtain higher mode coupling, which
is potential for greater sensitivity sensor. Combined with a long period fiber grating (LPFG), the cladding modes
promoted in fiber cladding are studied.
In some big power conditions, permanent deformation can be met on the irradiation points of the fiber. Although higher
loss is induced, there is also other advantage, such like high temperature stability. The sensitivity and stability of
temperature are discussed based on these sensors' configuration and mechanism. The experiences verify our laser
irritation sensors can survive in very high temperature. When coating with some gas absorption film such like zeolite
film, one reliable high sensitivity gas sensor is successfully demonstrated in low ppm vapor level.
Fiber inline core-cladding-mode interferometers (CCMI) fabricated by CO2 laser irradiations were demonstrated. CCMI
sensors operate based on the interference between the core mode and the cladding modes. Based on the way the
interferometer is configured, CCMI sensors can be categorized into two groups, namely the Mach-Zehnder
interferometer (MZI) type and the Michelson interferometer (MI) type. The MZI sensor works in transmission mode, i.e.,
the transmitted interference signal is detected. The MI sensor works in reflection mode, where the light passes the
interferometer twice and the reflected interference signal is detected. We conducted a temperature test and a refractive
index test to demonstrate their sensing capability.
Critical buildings such as hospitals and police stations must remain functional immediately following a major
earthquake event. Due to earthquake effects, they often experience large strains, leading to progressive collapses.
Therefore, monitoring and assessing the large strain condition of critical buildings is of paramount importance to
post-earthquake responses and evacuations in earthquake-prone regions. In this study, a novel large strain sensor
based on the long period fiber grating (LPFG) technology is proposed and developed. CO2 laser induced LPFG
sensors are characterized for such mechanical properties as strain sensitivity in extension and flexure, sensor
stability, and measurement range. For practical applications, the need for LPFG sensor packaging is identified and
verified in laboratory implementations. By introducing various strain transfer mechanisms, the strain sensitivity of
LPFG sensors can be customized for different applications at corresponding strain transfer ratios.
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