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Polycrystalline optical fiber amplifiers

Most of the studies to date on active IR fibers have concentrated on RE doping of glass fiber hosts. In fact, many of the studies on the RE-doped fluorides stem from the mid-1970s, when these materials were first developed. Very recently there has been some interest and research done on doping Ag-halides for possible use as OFAs. This effort has been largely led by Prof. Katzir and his group at Tel Aviv University. They have been able to grow mixed Ag-halide crystals with a host of RE and transition-metal elements, but most work has been done on Er³⁺, Pr³⁺, and Nd³⁺ doped into AgBrCl crystals.

After performing a series of measurements on RE dopants in the Ag halides, Oron et al.¹ focused on Nd³⁺ as the system to model for use as an OFA. They chose two configurations for their calculations and ultimately for fabrication into a PC fiber. The first was a single-clad 20-µm-core fiber and the second a doubleclad 20-µm-core, 100-µm first clad, surrounded by a second cladding. The doubleclad approach was chosen because this structure has been found to be a convenient method of increasing the pumping power without damage to the small core of an SM fiber. That is, higher powers are pumped into the larger-diameter first cladding, and as this power propagates down the fiber it transfers to the smaller core. Their results for the double-clad Ag-halide fiber are shown in Fig. 10.9. A sketch of a typical double-clad geometry is given in Fig. 10.9(a), and the results of their measurements using a 10-W pump beam for the Nd³⁺-doped (Nd³⁺ concentration of 10²⁵ m^-3) double-clad fiber are given in Fig. 10.9(b). The PC fiber had a core diameter of 20 µm and a first-cladding diameter of 200 µm. The data in Fig. 10.9(b) shows the amplification of 1.06-µm light from an input of 0.1W to a maximum of 4 W near the end of a 1.2-m-long fiber. In general, these results at 1.06 µm do not show the strong amplification normally seen in OFAs. One reason for this is the rather strong absorption by the undoped fiber at these short wavelengths. As may be seen from Fig. 6.10, the short-wavelength losses are rather high below about 3 µm. More work is underway to investigate other RE ions that have transitions in the 5-µm region, where these Ag-halide fibers have considerably lower loss.^2-4

Finally, there is a new and potentially interesting application for the Ag-halide fibers as fiber Bragg gratings. Uman et al.⁵ have illuminated Ag halide crystals with 353-nm light using a phase mask to produce a periodic structure on the surface of the crystal. The photolytic process leaves silver strips on the surface that are then etched off, leaving grooves as deep as 1.1 µm. They have been able to make a grating with a 100-µm period on the Ag-halide crystal using this technique.

Figure 10.9 (a) Configuration of double-clad Ag halide PC fiber for efficient pumping of light into the core and (b) pump attenuation and signal amplification for Nd3+-doped Ag halide double clad fiber.¹

References

1. R. Oron, D. Bunimovich, L. Nagli, A. Katzir, and A. A. Hardy, "Design considerations for rare earth doped silver halide fiber amplifiers," Opt. Eng., Vol. 40, pp. 1516-1520 (2001).
2. D. Bunimovich, L. Nagli, and A. Katzir, "Luminescence properties of praseodymium- and erbium-doped silver bromide crystals," Appl. Opt., Vol. 36, pp. 7708-7711 (1997).
3. D. Bunimovich, L. Nagli, and A. Katzir, "The visible and infrared luminescence of activated silver bromide crystals," Opt. Mater., Vol. 8, pp. 21-29 (1997).
4. L. Nagli, A. German, and A. Katzir, "Spectroscopic studies of Pr3+ ions in silver halide crystals," Appl. Opt., Vol. 39, pp. 5070-5075 (2000).
5. I. Uman, L. Nagli, Z. Barkay, F. Moser, and A. Katzir, "Photolytic fabrication of phase gratings on silver halide crystals," Appl. Opt., Vol. 41, pp. 4552-4556 (2002).