Hollow optical fiber

Abstract This chapter reviews the development of optical fiber probe Raman systems and their applications in life science and pharmaceutical studies. Especially, it is focused on miniaturized Raman probes which open new era in the spectroscopy of the life forms. The chapter also introduces the important optical properties of conventional optical fibers to use for Raman probes, as well as new types of optical fiber and devices, such as hollow optical fibers and photonic crystai fibers. 

A hollow optical fiber (HOF) is a totally different type of optical fiber [42 16]. Figure 2.3A shows the structure of an HOF. An HOF is made of a glass tube with an inner coating of silver and a dielectric film, which enhances reflectivity. Matsuura et al. reported that the root mean square surface roughness of the silver film was 12 nm in their fabricated HOF [44]. 

Fig. 3 Experimental scheme for a hollow optical fiber carrying a Bragg grating and a gold layer on the inner surface of the hollow fiber for waveguide mode-surface plasmon coupling and detection in transmission...

Hollow-Optical Fiber Probes for Bio-medical Spectroscopy... 

Shi YW, Ito K, Matsuura Y, Miyagi M (2005) Multiwavelength laser light transmission of hollow optical fiber from the visible to the mid-infrared. Opt Lett 30 2867-2869... 

Abstract Thin and flexible probes made with hollow-optical fibers may be useful for remote spectroscopy. Experimental results showed that these probes are useful for endoscopic measurements of infrared and Raman spectroscopy. A hollow-fiber probe has been used for remote FT-IR spectroscopy in the form of endoscopic measurement of infrared reflectometry spectra inside the body. This measurement was made possible by the hollow-fiber probe s flexibility, durability, nontoxicity, and low transmission loss. A hoUow-fiber probe with a ball lens at the end works as a confocal system for Raman spectroscopy. It can thus detect the molecular structure of biotissues with a high signal-to-noise ratio. Owing to their small diameter, the probes are useful for in vivo, noninvasive analysis using a flexible endoscope. 

Keywords Hollow-optical fibers Infrared spectroscopy Raman spectroscopy Endoscopes ET-IR Raman probes... 

A hollow-optical fiber is a prospective fiber-optic probe for infrared spectroscopy in medicine, owing to its nontoxicity and high mechanical and chemical stability [5]. However, it has been difficult to use it for remote spectroscopy because of its relatively high bending losses. Accordingly, the transmission efficiency should be improved by optimizing the fabrication conditions. 

In hollow-optical fibers for laser delivery, usually only some low order modes are excited because of small divergence angle of incident laser beam. In contrast, many high order modes are excited in hollow-optical fibers for spectroscopic applications because usually an incoherent light source like an arc lamp is used as the light source. Therefore, ray optic theory that can handle a wide divergence beam is used for evaluation of optical properties in this section. 

Figure 4 shows theoretical losses of (1) silver hollow-optical fiber, (2) dielectric-coated silver hollow-fiber with coating thickness of 0.39 pm, and (3) d = 0.66 pm. The thickness of 0.39 pm is the optimized value for 2 = 3 pm and 0.66 pm is the one for 2 = 5 pm. Parameters used in the calculation are rii = 1.53, z = 1 m, 2T = 1 mm, and cr = 0 and complex refractive index of silver is taken from literature [11]. In the calculation, a Gaussian beam with the divergence angle of 6° in full-width-half-maximum is assumed as an input beam. As seen from the calculated spectra, one can obtain a low loss region around the optimized wavelength. 

Figure 6 shows the measurement setup for remote infrared reflectometry using hollow-optical fibers. A Fourier-transfer infrared spectrometer with an external detector of HgCdTe was used in the experiment. Mid-infrared light from the... 

Fig. 6 Measurement setup for remote infrared reflectometry using hollow-optical fibers...

Harrington JA (2003) Infrared fibers and their applications. Chap 1. SPIE, Bellingham, WA Katagiri T, Komachi Y, Hattori Y, Matsuura Y, Miyagi M, Tashiro H, Sato H (2006) Hollow-optical fiber probe for confocal Raman endoscopy. Proc SPIE 6083 60830 Komachi Y, Sato H, Aizawa K, Tashiro H (2005) Micro-optical fiber probe for use in an intravascular Raman endoscope. Appl Opt 44 4722-4732... 

Figure 3. Schemes for coupling indicator phases with optical elements. B. Optical fiber coated on end A. Hollow optical fiber coated on cylindrical surface C. Flat optical waveguide coated with indicator phase.

Kim, J.Y., et al. Single-Cell-Level Cancer Therapy Using a Hollow Optical Fiber-Based Microplasma. Small 6(14), 1474—1478 (2010)... 

An example of evolution to a related science is atom photonics, in which the thermal motions of neutral atoms in a vacuum are controlled using optical nearfields [71]. Theoretical studies have examined single-atom manipulation based on the dressed photon model [72], and experimental studies have involved the first successful guidance of an atom through a hollow optical fiber [73]. Recent studies have examined atom-detecting devices [74], atom deflectors [75], and an atomic funnel [76]. Atom photonics will open up a new field of science that examines the interactions between dressed photons and single atoms. 

The main effects of laser-controlled atomic-beam optics are as follows (1) colli-mation of atomic beams, (2) reflection, (3) focusing, (4) guiding in hollow fibers, and (5) interferometry. The collimation of an atomic beam by making use of 2D laser cooling has already been considered in Section 5.4.2, and the laser guiding of atoms in a hollow optical fiber in Section 6.1.3. Therefore, we shall briefly consider the atomic reflection (atomic mirrors), atomic-beam focusing, and interferometry effects. In the latter case, laser radiation is used to produce the atomic beams necessary for the observation of the interference of atoms. 

Harris, D. J., and Savage, C. M. (1995). Atomic gravitational cavities from hollow optical fibers. Physical Review A, 51, 3967—3971. 

Ol shanii, M. A., Ovchinnikov, Yu. B., and Letokhov, V. S. (1993). Laser guiding of atoms in hollow optical fiber. Optics Communications, 98, 77-79. 

Renn, M. J., Donley, E. A., Cornell, E. A., Wieman, C. E., and Anderson, D. Z. (1996). Evanescent-wave guiding of atoms in hollow optical fibers. Physical Review A, 53, R648-R651. 

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