Akihito Hongo

Loss characteristics of circular hollow waveguides for incoherent infrared light

A simple and convenient method was proposed for the calculation of transmission loss in a circular hollow waveguide. The method is based on a simple ray theory and is applicable to multimode transmission of incoherent light. In order to confirm the validity of the method, spectral losses of various kinds of hollow waveguide were measured and compared with the theoretical values. For a silica waveguide, good coincidence was obtained between calculated and measured values, whereas, for metallic waveguides, surface irregularities must be taken into account for prediction of optical losses.

Fabrication of germanium‐coated nickel hollow waveguides for infrared transmission

Circular hollow nickel waveguides with an inner germanium layer are fabricated by using a method based on rf sputtering, plating, and etching techniques. Transmission losses less than 0.5 dB are achieved including launching losses for straight waveguides with 1.5 mmφ×1 m at 10.6‐μm wavelength. Bending losses of the waveguides are also examined.

Transmission of kilowatt-class CO 2 laser light through dielectric-coated metallic hollow waveguides for material processing

We have measured the transmittance of 3-kW CO(2) laser light through Ge-coated Ag hollow waveguides. The basic characteristics such as transmissivity, bending loss, and output-beam properties are described. A maximum laser power of 2.6 kW was delivered through a straight hollow waveguide that was 1.7 mm in diameter and 2 m long. Furthermore, 4-m-long waveguides were fabricated by joining two waveguides. Finally, preliminary experiments on welding steel plates were done with the light transmitted through the waveguide. Although the focusing properties of the output beam should be improved for practical laser processing, this type of waveguide is promising for high-power CO(2) laser-light transmission.

Transmission characteristics of dielectric-coated metallic waveguide for infrared transmission: Slab waveguide model

A metallic hollow waveguide with inner dielectric multilayers is proposed for the transmission of infrared light and its basic transmission characteristics are fully analyzed by using a two-dimensional slab waveguide model. The power loss in the waveguide is shown to be extremely reduced by coating dielectric layers properly. Simple loss formulas are also presented which consider absorptions of dielectric materials, and minimum possible losses are theoretically estimated. Finally, relations to bending losses in the waveguide are discussed.

Polyimide-coated small-diameter optical fiber sensors for embedding in composite laminate structures

We have studied health-monitoring systems using embedded optical fiber sensors for damage detection in FRP (Fiber Reinforced Plastics) composite laminates. For sensing of strain or temperature changes in these composite materials, we developed the FBG (Fiber Bragg Grating) sensors using the polyimide-coated small-diameter optical fiber with cladding diameter of 40 micrometers , which can be embedded in FRP composite laminates without inducing any structural defects. The FBG was subjected to high-temperature heat treatment at 300 degree(s)C in order to suppress the optical characteristics change of the FBG over a long period of time. Even after such high temperature treatment, the FBG retained sufficient mechanical strength and reflectivity. We embedded the FBG into FRP composite laminates and measured the optical characteristics against temperature and tensile strain. These experimental results show this type of the embedded FBG is very promising for practical health-monitoring systems. In addition, we have investigated various multipoint measuring systems and propose the time division multiplexing system using these FBG sensors.

Organic solid-state distributed feedback dye laser with a nonmorphological modification grating

We report an organic solid-state distributed feedback laser consisting of an organic active layer and a Bragg grating without morphological change. The active layer consists of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran as a laser dye, tris(8hydroxyquinoline) aluminum as a host, and poly(methyl methacrylate) as a binder. The threshold and linewidth of the laser are 400 μJ/cm2 and 1.5 nm, respectively. With this laser, the Bragg grating exists out of the active layer and the grating surface is flat, which are very important for device fabrication and electric driving of the laser.

Development of small-diameter optical fiber sensors for damage detection in composite laminates

Optical fiber sensor systems have been recently used for ,health monitoring of fiber reinforced plastic (FRP) composite laminate structures. However, the diameter of optical fibers for optical communication is normally 125 μm, which is similar to the thickness of one ply in FRP composite laminates. In order to embed optical fibers within one ply of FRP without introducing any defects in laminates, small- diameter optical fibers are necessary. The present paper describes development of small-diameter optical fiber sensors for damage detection in composite laminates. The coating diameter of the prototype small-diameter optical fiber is 52 μm, while the diameters of the cladding and the core are 40 μm and 8.5 μm, respectively. FBG (Fiber Bragg Grating) sensors were also fabricated with these fibers. Gratings were inscribed into the optical fiber by ultra-violet irradiation method. The wavelength shift and the response by strain of FBG itself were measured. The strain response, when small-diameter optical fiber and FBG sensors were embedded in composite laminates, were evaluated for practical application to detect the damage evolution.

Excitation dependent losses and temperature increase in various hollow waveguides at 10.6 μm

Abstract Theoretical and experimental studies have been made on excitation dependent transmission properties of various hollow waveguides at 10.6 μm. A heat problem of the waveguide has been experimentally treated when high powered CO 2 laser light is launched.

Transmission of 1 kW‐class CO2 laser light through circular hollow waveguides for material processing

For the purpose of material processing, we have studied transmission characteristics of germanium‐coated silver hollow waveguides with 1.5 mm φ×80 cm. The maximum input power was 1 kW and the transmission was 90% under straight condition. Steel plates 1.6 mm thick were cut by CO2 laser light through the hollow waveguide and the authors confirmed that this type of waveguide could deliver sufficient laser energy for material processing.

Loss reduction of dielectric-coated metallic hollow waveguides for CO2 laser light transmission

Abstract A condition for reducing transmission loss of a dielectric-coated metallic hollow waveguide is shown theoretically when the inner metallic surface is not perfect. It is also shown that the transmission loss for CO 2 laser light is 10.6 μm wavelength is able to be estimated from an attenuation spectrum of a short waveguide. By fabricating a ZnSe coated Ag hollow waveguide of short length, we have shown that it has a lower loss than a Ge coated waveguide for CO 2 laser light.