Active and passive chalcogenide glass optical fibers for IR applications: a review

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Abstract

Chalcogenide glass fibers based on sulfide, selenide, telluride and their rare earth doped compositions are being actively investigated worldwide. Great strides have been made in reducing optical losses using improved chemical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite these problems, current single mode and multimode chalcogenide glass fibers are enabling numerous applications. Some of these applications include laser power delivery, chemical sensing, imaging, scanning near field microscopy/spectroscopy, fiber IR sources/lasers, amplifiers and optical switches. The authors assert that the research and development of chalcogenide glasses will grow in the foreseeable future, especially with respect to improvements in the optical quality of the fibers and the performance of the fibers in existing and future applications.

Introduction

Chalcogenide glasses are based on the chalcogen elements S, Se and Te and the addition of other elements such as Ge, As and Sb leads to the formation of stable glasses [1]. The addition of halides leads to the formation of chalcohalide glasses [2]. Examples of stable glasses include As2S3 [1], Ge20S40Br40 [2], As2Se3 [1]and Ge30As10Se30Te30 [3]. More recent efforts have reported on rare earth doping for active applications and consequently alternative glasses have been developed. Examples of these glass systems include Ge–Ga–S [4], Ge–As–Ga–S [5], Ga–La–S [6], Ga–Na–S [7], Ge–S–I [8]and Ge–As–Se [9].

Since the chalcogenide glass fibers transmit into the IR, there are numerous potential applications in the civil, medical and military areas. These can be essentially divided into two groups, namely `passive' and `active' applications. The passive applications utilize chalcogenide fibers as a light conduit from one location to another without changing the optical properties, other than that due to scattering, absorption and end face reflection losses associated with the fiber. Active applications of chalcogenide glass fibers are where the initial light propagating through the fiber is modified by a process other than that due to scattering, absorption and end face reflection losses associated with the fiber. Examples of these include fiber lasers, amplifiers, bright sources, gratings and nonlinear effects.

This paper describes some of the applications being developed in our laboratory as well as a review of the literature which describe where chalcogenide fibers are being used and where they could potentially be used.

Section snippets

Experimental techniques for preparing fibers

Chalcogenide glasses are either melted directly in quartz ampoules or in vitreous carbon crucibles located within quartz ampoules. Typical melt temperatures range from 600°C to 1100°C, depending upon composition. The liquids are quenched and the glass rods annealed at temperatures around the appropriate softening temperatures. The optical fibers are obtained by heating preforms fabricated via rod-in-tube type processes 10, 11or by double crucible (DC) processes (Fig. 1) 11, 12, 13. The cladding

Properties of fibers

Table 1 lists some physical, mechanical and optical properties of two chalcogenide glasses used in making optical fibers [17]. Compared to the more traditional oxide glasses, they can be described as having lower Tgs, larger coefficients of thermal expansion, smaller hardness and higher indices of refraction [17]. From a practical viewpoint, the most important difference is their transmission at wavelengths >5 μm. Fig. 2(a) and (b) show the transmission spectra of three chalcogenide fibers made

Laser power delivery

High power CO and CO2 lasers operating at 5.4 and 10.6 μm, respectively, are available and are used for industrial welding and cutting. Transmitting the laser power through fibers enables remote operation. To-date, Te based fibers have demonstrated output powers of 10.7 W for 19.4 W launched power (efficiency=55.2%) at 10.6 μm [18]. The fibers possessed a PbF2 antireflection (AR) coating and were cooled with water to prevent thermal lensing caused by their increase in absorption coefficient

Conclusions

Progress has been made in reducing the optical losses of the chalcogenide glass fibers in the past several years and made possible numerous applications. We assume that IR fiber optics will become increasingly more important in the future as further improvements are made to the quality of the fibers and new compositions developed. One of the most exciting developments in the future is going to be in the area of rare earth ion doping of fibers for IR fluorescence emission. The IR light sources,

Acknowledgements

The authors acknowledge L.E. Busse, L.B. Shaw, D. Schaafsma, P. Pureza, V.Q. Nyugen, R. Mossadegh, B. Cole, R. Miklos, F. Kung and B. Harbison.

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