Annular Cladding Research Articles

Investigation into the mechanical behavior of a polymer optical fiber embedded in a structural adhesive

To guaranty a safe load transfer through adhesively bonded joints, Structural Health Monitoring (SHM) methods that allow for permanent monitoring of the structural bond condition, are becoming more and more important. To meet this increasing demand, a novel approach for SHM of adhesively bonded joints by use of integrated Polymer Optical Fibers (POF) has been demonstrated in previous work. The sensor operating principle is based on strain transfer from the structural adhesive to the integrated POF. This strain transfer leads to a deformation of the POF cross-sectional shape and in that way affects the optical light propagation in the POF. In the current work, numerical and experimental results are presented into the mechanical behavior of embedded POF in adhesive bulk specimens subjected to tensile loading. Local deformations of both the POF (Asahi Kasei DB 500) and the structural adhesive (3 M Scotch Weld DP410) are evaluated using Digital Image Correlation (DIC). In the loading direction, a linear correlation is shown between strain inside the POF core and strain of the surrounding adhesive. Tests results, as well as fracture analysis, show a highly ductile behavior and failure of the POF cladding material and the interface between the POF core and annular cladding.

Study on powder dynamics of ultrahigh-speed laser cladding

In view of the key technical problems of ultrahigh-speed laser cladding, this paper has studied the influence of different laser power, powder feeding rate, and carrier gas velocity on the formation of the cladding layer. A theoretical model of the powder flow field in the laser cladding process has been established. An energy attenuation algorithm based on the powder shading ratio has been designed, and the particle thermal field has been analyzed. We verified their feasibility with four radially symmetrical powder flow nozzles and extended them to the annular cladding nozzles. It is found that the powder spot temperature can be increased by increasing the laser power and setting powder focus point 0–2 mm above the laser focus point, and the utilization of laser energy in powder can be improved by reducing the particle size and increasing the powder feeding rate and carrier gas velocity. The results provide theoretical references for the application of ultrahigh-speed laser cladding on the surface strengthening of shaft components.

Temperature Sensor in Suspended Core Hollow Fiber Covered With Reduced Graphene Oxide

A compact temperature sensor based on a Mach–Zehnder interferometer (MZI) is demonstrated in a suspended core hollow fiber (SCHF) covered with reduced graphene oxide (rGO). The MZI is constructed by connecting two single-mode fibers with an intermediate section SCHF in offset splicing, which introduces the interference between the fundamental and high-order modes of the suspended core. The SCHF-based MZI without rGO is insensitive to the temperature; however, after employing rGO into the SCHF, the temperature sensitivity of ~179.4 pm/°C in the range of 30 °C–80 °C is obtained experimentally. Compared to microfiber-based sensors, the annular cladding can prevent the sensor from external environmental pollution. The proposed rGO-based all-fiber temperature sensor has good mechanical strength and is well compatible with fiber-optic systems, which makes it a good candidate for temperature monitoring. Meanwhile, the air hole of the SCHF is convenient for forming microfluidic channel to realize gas concentration or humidity sensing.

Annular waveguide lasers at 1064 nm in Nd:YAG crystal produced by femtosecond laser inscription.

We report on the fabrication of annular cladding waveguides in Nd:YAG laser crystal by using femtosecond laser inscription. The circular cross sections of the optical waveguides were in dual-cladding and tri-cladding shapes with different diameters. Under the optical pump at 808nm, the generated continuous-wave lasers at 1064nm have been realized in the annular cladding waveguides. For dual-cladding and tri-cladding waveguides, the slope efficiencies of the waveguide lasers were calculated to be 21.3% and 20%, and the maximum output powers measured were 82mW and 84mW, respectively. In addition, the annular modal profiles were obtained for these cladding waveguides.

In-fiber integrated gas pressure sensor based on a hollow optical fiber with two cores

We demonstrate an in-fiber integrated highly-sensitive gas pressure sensor based on a special designed hollow eccentric twin-core optical fiber (HTCF). In this design, the core in the hole and the core in the annular cladding of the HTCF are separately used as sensing arm and reference arm. Then, in-fiber gas pressure detection can be first realized in the long hole of the HTCF. The experimental results show that the sensor exhibits a gas pressure sensitivity of 0.42 nm/bar, with low temperature sensitivity of −51 pm/℃, and a maximum bending sensitivity of 98 pm/μm. Comparing with traditional optical fiber gas pressure sensors, this device first realizes in-fiber gas pressure detecting through the special designed optical fiber with a compact size which can be easily connected and integrated in practical applications.

Optofluidic in-fiber interferometer based on hollow optical fiber with two cores.

We demonstrate a novel integrated optical fiber interferometer for in-fiber optofluidic detection. It is composed of a specially designed hollow optical fiber with a micro-channel and two cores. One core on the inner surface of the micro-channel is served as sensing arm and the other core in the annular cladding is served as reference arm. Fusion-and-tapering method is employed to couple light from a single mode fiber to the hollow optical fiber in this device. Sampling is realized by side opening a microhole on the surface of the hollow optical fiber. Under differential pressure between the end of the hollow fiber and the microhole, the liquids can form steady microflows in the micro-channel. Simultaneously, the interference spectrum of the interferometer device shifts with the variation of the concentration of the microfluid in the channel. The optofluidic in-fiber interferometer has a sensitivity of refractive index around 2508 nm/RIU for NaCl. For medicine concentration detection, its sensitivity is 0.076 nm/mmolL-1 for ascorbic acid. Significantly, this work presents a compact microfluidic in-fiber interferometer with a micro-channel which can be integrated with chip devices without spatial optical coupling and without complex manufacturing procedure of the waveguide on the chips.

Annular-cladding erbium doped multicore fiber for SDM amplification.

We propose and numerically investigate annular-cladding erbium doped multicore fibers (AC-EDMCF) with either solid or air hole inner cladding to enhance the pump power efficiency in optical amplifiers for spatial division multiplexing (SDM) transmission links. We first propose an all-glass fiber in which a central inner cladding region with a depressed refractive index is introduced to confine the pump inside a ring-shaped region overlapping the multiple signal cores. Through numerical simulations, we determine signal core and annular pump cladding parameters respecting fabrication constraints. We also propose and examine a multi-spot injection scheme for launching the pump in the annular cladding. With this all-glass fiber with annular cladding, our results predict 10 dB increase in gain and 21% pump power savings compared to the standard double cladding design. We also investigate a fiber with an air hole inner cladding to further enhance the pump power confinement and minimize power leaking into the inner cladding. The results are compared to the all-glass AC-EDMCF.

Characteristics of embedded-core hollow optical fiber

We propose a novel embedded-core hollow optical fiber composed of a central air hole, a semi-elliptical core, and an annular cladding. The fiber characteristics are investigated based on the finite element method (FEM), including mode properties, birefringence, confinement loss, evanescent field and bending loss. The results reveal that the embedded-core hollow optical fiber has a non-zero cut-off frequency for the fundamental mode. The birefringence of the hollow optical fiber is insensitive to the size of the central air hole and ultra-sensitive to the thickness of the cladding between the core and the air hole. Both thin cladding between the core and the air hole and small core ellipticity lead to high birefringence. An ultra-low birefringence fiber can be achieved by selecting a proper ellipticity of the core. The embedded-core hollow optical fiber holds a strong evanescent field due to special structure of thin cladding and therefore it is of importance for potential applications such as gas and biochemical sensors. The bending losses are measured experimentally. The bending loss strongly depends on bending orientations of the fiber. The proposed fiber can be used as polarization interference devices if the orientation angle of the fiber core is neither 0° nor 90°.