An overview of ultrasound fiber guides, their transmission properties, and their applications in sensing is presented.
Ultrasound fiber guides are structures similar to optical fibers that are used for transmission of acoustic waves. They
consist of a core region surrounded by a cladding layer to help confine the wave to the core. Ultrasound fiber guides
may be fabricated with glass materials such as pure and doped fused silica, using fiber optics manufacturing
technology. The underlying principles of fiber optic sensing in many cases are also applicable to ultrasound fiber
guides, hence the potential applications of these waveguides in sensing and health monitoring of infrastructures.
Propagation properties of ultrasound fiber guides are reviewed. Attention is focused on guides with small
differences between the parameters of the core and cladding, often a necessary requirement for single-mode
operation. Various types of guided modes including flexural, torsional, and radial modes are discussed. These modes
are predominantly shear type. Ultrasound fiber guides also support another group of modes with complex
propagation constants, which are referred to as leaky longitudinal modes. These modes lose power as propagate
along the guide through radiation. Similarities and differences between optical and ultrasound modes in fiber guides
are addressed.
We present exact analytical solutions to the ideal one-dimensional planar photonic crystal waveguides (1-D PPCWs) that consist of a central core and infinite number of cladding layers symmetrically placed around the core. We show that these exact solutions allow one to distinguish clearly between light guidance due to total internal reflection and light guidance due to the photonic crystal effect. We also compare structures with finite and infinite cladding layers and provide results for the propagation characteristics and the modal field distributions.
Tapered photonic crystal fibers (PCFs), similar to tapered conventional fibers, can be either adiabatic of nonadiabatic. Here, we examine the adiabaticity of the PCF taper and obtain three-dimensional plots showing the evolution of power flow inside the tapered PCF. Also, we analyze different taper shapes, such as the linear taper, raised-cosine taper, and the modified exponential taper. Other factors, such as the taper length and the number of air hole rings are also investigated.
In this paper, we introduce a novel optical fiber, referred to as Fresnel fiber. This fiber has equal area rings and is fabricated using two materials only. Transmission properties of Fresnel fibers, including axial propagation constant, effective area, and dispersion, are investigated. Design of these fibers for large or small effective area and prescribed dispersion characteristics, required for sensing or communication applications, are also addressed.
Recently developed optical fibers rely on an array of air holes in the cladding to confine light to the fiber core as opposed to conventional telecommunications fibers that require a refractive index difference produced by different composition glasses in the core and cladding regions. Holey fibers have been fabricated by drawing an array of tubes stacked around a solid central core. In this paper, we describe a new technique to produce the holes (or pores) in the cladding region. These new fibers have been made by drawing a preform, consisting of a porous outer cladding region surrounding a solid central core region, into a fiber. During the fiber drawing process, the pores initially present in the preform cladding region are drawn into small, long, thin tubular pores. Controlling the dimensions and distribution of the pores in the preform can control the physical dimensions and distribution of the pores in the fiber. In some of the preforms, the porous cladding region in the preform was prepared by sol gel techniques. The preform fabrication process and fiber drawing process used to produce these new holey fibers as well as the results of the morphological study elucidating the size, shape and distribution of the porous phase are presented.
Refractive-index nonlinearities have negligible effect on the performance of short-haul fiber-optic communication links utilizing electronic repeaters. However, in long optical fiber links, nonlinearities can cause severe signal degradations. To mitigate nonlinear effects, new generation of fibers, referred to as large effective-area fibers, have been introduced in recent years. This paper reviews the latest research and development work on these fibers conducted by several research groups around the world. Attention is focused on a class of large effective-area fibers that are based on a depressed-core multiple-cladding design. Transmission properties, including dispersion, dispersion slope, effective area, mode-field diameter, bending loss, polarization-mode dispersion, and cutoff wavelength are discussed. Dispersion-shifted, non-zero dispersion-shifted, and dispersion-flattened designs are addressed. Design optimization, particularly with regard to effective area, bending loss, and polarization-mode dispersion, is elaborated upon. The trade-off between effective-area and bending loss is emphasized. Results for dispersion-shifted and non-zero dispersion-shifted large effective-area fibers with zero polarization-mode dispersion and low bending loss at 1.55 micrometer wavelength are presented.
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